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+@c -*-texinfo-*-
+@c This is part of the GNU Emacs Lisp Reference Manual.
+@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001,
+@c   2002, 2003, 2004, 2005, 2006, 2007  Free Software Foundation, Inc.
+@c See the file elisp.texi for copying conditions.
+@setfilename ../info/modes
+@node Modes, Documentation, Keymaps, Top
+@chapter Major and Minor Modes
+@cindex mode
+
+  A @dfn{mode} is a set of definitions that customize Emacs and can be
+turned on and off while you edit.  There are two varieties of modes:
+@dfn{major modes}, which are mutually exclusive and used for editing
+particular kinds of text, and @dfn{minor modes}, which provide features
+that users can enable individually.
+
+  This chapter describes how to write both major and minor modes, how to
+indicate them in the mode line, and how they run hooks supplied by the
+user.  For related topics such as keymaps and syntax tables, see
+@ref{Keymaps}, and @ref{Syntax Tables}.
+
+@menu
+* Hooks::              How to use hooks; how to write code that provides hooks.
+* Major Modes::        Defining major modes.
+* Minor Modes::        Defining minor modes.
+* Mode Line Format::   Customizing the text that appears in the mode line.
+* Imenu::              How a mode can provide a menu
+                         of definitions in the buffer.
+* Font Lock Mode::     How modes can highlight text according to syntax.
+* Desktop Save Mode::  How modes can have buffer state saved between
+                         Emacs sessions.
+@end menu
+
+@node Hooks
+@section Hooks
+@cindex hooks
+
+  A @dfn{hook} is a variable where you can store a function or functions
+to be called on a particular occasion by an existing program.  Emacs
+provides hooks for the sake of customization.  Most often, hooks are set
+up in the init file (@pxref{Init File}), but Lisp programs can set them also.
+@xref{Standard Hooks}, for a list of standard hook variables.
+
+@cindex normal hook
+  Most of the hooks in Emacs are @dfn{normal hooks}.  These variables
+contain lists of functions to be called with no arguments.  By
+convention, whenever the hook name ends in @samp{-hook}, that tells
+you it is normal.  We try to make all hooks normal, as much as
+possible, so that you can use them in a uniform way.
+
+  Every major mode function is supposed to run a normal hook called
+the @dfn{mode hook} as the one of the last steps of initialization.
+This makes it easy for a user to customize the behavior of the mode,
+by overriding the buffer-local variable assignments already made by
+the mode.  Most minor mode functions also run a mode hook at the end.
+But hooks are used in other contexts too.  For example, the hook
+@code{suspend-hook} runs just before Emacs suspends itself
+(@pxref{Suspending Emacs}).
+
+  The recommended way to add a hook function to a normal hook is by
+calling @code{add-hook} (see below).  The hook functions may be any of
+the valid kinds of functions that @code{funcall} accepts (@pxref{What
+Is a Function}).  Most normal hook variables are initially void;
+@code{add-hook} knows how to deal with this.  You can add hooks either
+globally or buffer-locally with @code{add-hook}.
+
+@cindex abnormal hook
+  If the hook variable's name does not end with @samp{-hook}, that
+indicates it is probably an @dfn{abnormal hook}.  That means the hook
+functions are called with arguments, or their return values are used
+in some way.  The hook's documentation says how the functions are
+called.  You can use @code{add-hook} to add a function to an abnormal
+hook, but you must write the function to follow the hook's calling
+convention.
+
+  By convention, abnormal hook names end in @samp{-functions} or
+@samp{-hooks}.  If the variable's name ends in @samp{-function}, then
+its value is just a single function, not a list of functions.
+
+  Here's an example that uses a mode hook to turn on Auto Fill mode when
+in Lisp Interaction mode:
+
+@example
+(add-hook 'lisp-interaction-mode-hook 'turn-on-auto-fill)
+@end example
+
+  At the appropriate time, Emacs uses the @code{run-hooks} function to
+run particular hooks.
+
+@defun run-hooks &rest hookvars
+This function takes one or more normal hook variable names as
+arguments, and runs each hook in turn.  Each argument should be a
+symbol that is a normal hook variable.  These arguments are processed
+in the order specified.
+
+If a hook variable has a non-@code{nil} value, that value should be a
+list of functions.  @code{run-hooks} calls all the functions, one by
+one, with no arguments.
+
+The hook variable's value can also be a single function---either a
+lambda expression or a symbol with a function definition---which
+@code{run-hooks} calls.  But this usage is obsolete.
+@end defun
+
+@defun run-hook-with-args hook &rest args
+This function is the way to run an abnormal hook and always call all
+of the hook functions.  It calls each of the hook functions one by
+one, passing each of them the arguments @var{args}.
+@end defun
+
+@defun run-hook-with-args-until-failure hook &rest args
+This function is the way to run an abnormal hook until one of the hook
+functions fails.  It calls each of the hook functions, passing each of
+them the arguments @var{args}, until some hook function returns
+@code{nil}.  It then stops and returns @code{nil}.  If none of the
+hook functions return @code{nil}, it returns a non-@code{nil} value.
+@end defun
+
+@defun run-hook-with-args-until-success hook &rest args
+This function is the way to run an abnormal hook until a hook function
+succeeds.  It calls each of the hook functions, passing each of them
+the arguments @var{args}, until some hook function returns
+non-@code{nil}.  Then it stops, and returns whatever was returned by
+the last hook function that was called.  If all hook functions return
+@code{nil}, it returns @code{nil} as well.
+@end defun
+
+@defun add-hook hook function &optional append local
+This function is the handy way to add function @var{function} to hook
+variable @var{hook}.  You can use it for abnormal hooks as well as for
+normal hooks.  @var{function} can be any Lisp function that can accept
+the proper number of arguments for @var{hook}.  For example,
+
+@example
+(add-hook 'text-mode-hook 'my-text-hook-function)
+@end example
+
+@noindent
+adds @code{my-text-hook-function} to the hook called @code{text-mode-hook}.
+
+If @var{function} is already present in @var{hook} (comparing using
+@code{equal}), then @code{add-hook} does not add it a second time.
+
+It is best to design your hook functions so that the order in which they
+are executed does not matter.  Any dependence on the order is ``asking
+for trouble.''  However, the order is predictable: normally,
+@var{function} goes at the front of the hook list, so it will be
+executed first (barring another @code{add-hook} call).  If the optional
+argument @var{append} is non-@code{nil}, the new hook function goes at
+the end of the hook list and will be executed last.
+
+@code{add-hook} can handle the cases where @var{hook} is void or its
+value is a single function; it sets or changes the value to a list of
+functions.
+
+If @var{local} is non-@code{nil}, that says to add @var{function} to
+the buffer-local hook list instead of to the global hook list.  If
+needed, this makes the hook buffer-local and adds @code{t} to the
+buffer-local value.  The latter acts as a flag to run the hook
+functions in the default value as well as in the local value.
+@end defun
+
+@defun remove-hook hook function &optional local
+This function removes @var{function} from the hook variable
+@var{hook}.  It compares @var{function} with elements of @var{hook}
+using @code{equal}, so it works for both symbols and lambda
+expressions.
+
+If @var{local} is non-@code{nil}, that says to remove @var{function}
+from the buffer-local hook list instead of from the global hook list.
+@end defun
+
+@node Major Modes
+@section Major Modes
+@cindex major mode
+
+  Major modes specialize Emacs for editing particular kinds of text.
+Each buffer has only one major mode at a time.  For each major mode
+there is a function to switch to that mode in the current buffer; its
+name should end in @samp{-mode}.  These functions work by setting
+buffer-local variable bindings and other data associated with the
+buffer, such as a local keymap.  The effect lasts until you switch
+to another major mode in the same buffer.
+
+@menu
+* Major Mode Basics::
+* Major Mode Conventions::  Coding conventions for keymaps, etc.
+* Auto Major Mode::         How Emacs chooses the major mode automatically.
+* Mode Help::               Finding out how to use a mode.
+* Derived Modes::           Defining a new major mode based on another major
+                              mode.
+* Generic Modes::           Defining a simple major mode that supports
+                              comment syntax and Font Lock mode.
+* Mode Hooks::              Hooks run at the end of major mode functions.
+* Example Major Modes::     Text mode and Lisp modes.
+@end menu
+
+@node Major Mode Basics
+@subsection Major Mode Basics
+@cindex Fundamental mode
+
+  The least specialized major mode is called @dfn{Fundamental mode}.
+This mode has no mode-specific definitions or variable settings, so each
+Emacs command behaves in its default manner, and each option is in its
+default state.  All other major modes redefine various keys and options.
+For example, Lisp Interaction mode provides special key bindings for
+@kbd{C-j} (@code{eval-print-last-sexp}), @key{TAB}
+(@code{lisp-indent-line}), and other keys.
+
+  When you need to write several editing commands to help you perform a
+specialized editing task, creating a new major mode is usually a good
+idea.  In practice, writing a major mode is easy (in contrast to
+writing a minor mode, which is often difficult).
+
+  If the new mode is similar to an old one, it is often unwise to
+modify the old one to serve two purposes, since it may become harder
+to use and maintain.  Instead, copy and rename an existing major mode
+definition and alter the copy---or use @code{define-derived-mode} to
+define a @dfn{derived mode} (@pxref{Derived Modes}).  For example,
+Rmail Edit mode is a major mode that is very similar to Text mode
+except that it provides two additional commands.  Its definition is
+distinct from that of Text mode, but uses that of Text mode.
+
+  Even if the new mode is not an obvious derivative of any other mode,
+it is convenient to use @code{define-derived-mode} with a @code{nil}
+parent argument, since it automatically enforces the most important
+coding conventions for you.
+
+  For a very simple programming language major mode that handles
+comments and fontification, you can use @code{define-generic-mode}.
+@xref{Generic Modes}.
+
+  Rmail Edit mode offers an example of changing the major mode
+temporarily for a buffer, so it can be edited in a different way (with
+ordinary Emacs commands rather than Rmail commands).  In such cases, the
+temporary major mode usually provides a command to switch back to the
+buffer's usual mode (Rmail mode, in this case).  You might be tempted to
+present the temporary redefinitions inside a recursive edit and restore
+the usual ones when the user exits; but this is a bad idea because it
+constrains the user's options when it is done in more than one buffer:
+recursive edits must be exited most-recently-entered first.  Using an
+alternative major mode avoids this limitation.  @xref{Recursive
+Editing}.
+
+  The standard GNU Emacs Lisp library directory tree contains the code
+for several major modes, in files such as @file{text-mode.el},
+@file{texinfo.el}, @file{lisp-mode.el}, @file{c-mode.el}, and
+@file{rmail.el}.  They are found in various subdirectories of the
+@file{lisp} directory.  You can study these libraries to see how modes
+are written.  Text mode is perhaps the simplest major mode aside from
+Fundamental mode.  Rmail mode is a complicated and specialized mode.
+
+@node Major Mode Conventions
+@subsection Major Mode Conventions
+@cindex major mode conventions
+@cindex conventions for writing major modes
+
+  The code for existing major modes follows various coding conventions,
+including conventions for local keymap and syntax table initialization,
+global names, and hooks.  Please follow these conventions when you
+define a new major mode.  (Fundamental mode is an exception to many
+of these conventions, because its definition is to present the global
+state of Emacs.)
+
+  This list of conventions is only partial, because each major mode
+should aim for consistency in general with other Emacs major modes.
+This makes Emacs as a whole more coherent.  It is impossible to list
+here all the possible points where this issue might come up; if the
+Emacs developers point out an area where your major mode deviates from
+the usual conventions, please make it compatible.
+
+@itemize @bullet
+@item
+Define a command whose name ends in @samp{-mode}, with no arguments,
+that switches to the new mode in the current buffer.  This command
+should set up the keymap, syntax table, and buffer-local variables in an
+existing buffer, without changing the buffer's contents.
+
+@item
+Write a documentation string for this command that describes the
+special commands available in this mode.  @kbd{C-h m}
+(@code{describe-mode}) in your mode will display this string.
+
+The documentation string may include the special documentation
+substrings, @samp{\[@var{command}]}, @samp{\@{@var{keymap}@}}, and
+@samp{\<@var{keymap}>}, which enable the documentation to adapt
+automatically to the user's own key bindings.  @xref{Keys in
+Documentation}.
+
+@item
+The major mode command should start by calling
+@code{kill-all-local-variables}.  This runs the normal hook
+@code{change-major-mode-hook}, then gets rid of the buffer-local
+variables of the major mode previously in effect.  @xref{Creating
+Buffer-Local}.
+
+@item
+The major mode command should set the variable @code{major-mode} to the
+major mode command symbol.  This is how @code{describe-mode} discovers
+which documentation to print.
+
+@item
+The major mode command should set the variable @code{mode-name} to the
+``pretty'' name of the mode, as a string.  This string appears in the
+mode line.
+
+@item
+@cindex functions in modes
+Since all global names are in the same name space, all the global
+variables, constants, and functions that are part of the mode should
+have names that start with the major mode name (or with an abbreviation
+of it if the name is long).  @xref{Coding Conventions}.
+
+@item
+In a major mode for editing some kind of structured text, such as a
+programming language, indentation of text according to structure is
+probably useful.  So the mode should set @code{indent-line-function}
+to a suitable function, and probably customize other variables
+for indentation.
+
+@item
+@cindex keymaps in modes
+The major mode should usually have its own keymap, which is used as the
+local keymap in all buffers in that mode.  The major mode command should
+call @code{use-local-map} to install this local map.  @xref{Active
+Keymaps}, for more information.
+
+This keymap should be stored permanently in a global variable named
+@code{@var{modename}-mode-map}.  Normally the library that defines the
+mode sets this variable.
+
+@xref{Tips for Defining}, for advice about how to write the code to set
+up the mode's keymap variable.
+
+@item
+The key sequences bound in a major mode keymap should usually start with
+@kbd{C-c}, followed by a control character, a digit, or @kbd{@{},
+@kbd{@}}, @kbd{<}, @kbd{>}, @kbd{:} or @kbd{;}.  The other punctuation
+characters are reserved for minor modes, and ordinary letters are
+reserved for users.
+
+A major mode can also rebind the keys @kbd{M-n}, @kbd{M-p} and
+@kbd{M-s}.  The bindings for @kbd{M-n} and @kbd{M-p} should normally
+be some kind of ``moving forward and backward,'' but this does not
+necessarily mean cursor motion.
+
+It is legitimate for a major mode to rebind a standard key sequence if
+it provides a command that does ``the same job'' in a way better
+suited to the text this mode is used for.  For example, a major mode
+for editing a programming language might redefine @kbd{C-M-a} to
+``move to the beginning of a function'' in a way that works better for
+that language.
+
+It is also legitimate for a major mode to rebind a standard key
+sequence whose standard meaning is rarely useful in that mode.  For
+instance, minibuffer modes rebind @kbd{M-r}, whose standard meaning is
+rarely of any use in the minibuffer.  Major modes such as Dired or
+Rmail that do not allow self-insertion of text can reasonably redefine
+letters and other printing characters as special commands.
+
+@item
+Major modes modes for editing text should not define @key{RET} to do
+anything other than insert a newline.  However, it is ok for
+specialized modes for text that users don't directly edit, such as
+Dired and Info modes, to redefine @key{RET} to do something entirely
+different.
+
+@item
+Major modes should not alter options that are primarily a matter of user
+preference, such as whether Auto-Fill mode is enabled.  Leave this to
+each user to decide.  However, a major mode should customize other
+variables so that Auto-Fill mode will work usefully @emph{if} the user
+decides to use it.
+
+@item
+@cindex syntax tables in modes
+The mode may have its own syntax table or may share one with other
+related modes.  If it has its own syntax table, it should store this in
+a variable named @code{@var{modename}-mode-syntax-table}.  @xref{Syntax
+Tables}.
+
+@item
+If the mode handles a language that has a syntax for comments, it should
+set the variables that define the comment syntax.  @xref{Options for
+Comments,, Options Controlling Comments, emacs, The GNU Emacs Manual}.
+
+@item
+@cindex abbrev tables in modes
+The mode may have its own abbrev table or may share one with other
+related modes.  If it has its own abbrev table, it should store this
+in a variable named @code{@var{modename}-mode-abbrev-table}.  If the
+major mode command defines any abbrevs itself, it should pass @code{t}
+for the @var{system-flag} argument to @code{define-abbrev}.
+@xref{Defining Abbrevs}.
+
+@item
+The mode should specify how to do highlighting for Font Lock mode, by
+setting up a buffer-local value for the variable
+@code{font-lock-defaults} (@pxref{Font Lock Mode}).
+
+@item
+The mode should specify how Imenu should find the definitions or
+sections of a buffer, by setting up a buffer-local value for the
+variable @code{imenu-generic-expression}, for the two variables
+@code{imenu-prev-index-position-function} and
+@code{imenu-extract-index-name-function}, or for the variable
+@code{imenu-create-index-function} (@pxref{Imenu}).
+
+@item
+The mode can specify a local value for
+@code{eldoc-documentation-function} to tell ElDoc mode how to handle
+this mode.
+
+@item
+Use @code{defvar} or @code{defcustom} to set mode-related variables, so
+that they are not reinitialized if they already have a value.  (Such
+reinitialization could discard customizations made by the user.)
+
+@item
+@cindex buffer-local variables in modes
+To make a buffer-local binding for an Emacs customization variable, use
+@code{make-local-variable} in the major mode command, not
+@code{make-variable-buffer-local}.  The latter function would make the
+variable local to every buffer in which it is subsequently set, which
+would affect buffers that do not use this mode.  It is undesirable for a
+mode to have such global effects.  @xref{Buffer-Local Variables}.
+
+With rare exceptions, the only reasonable way to use
+@code{make-variable-buffer-local} in a Lisp package is for a variable
+which is used only within that package.  Using it on a variable used by
+other packages would interfere with them.
+
+@item
+@cindex mode hook
+@cindex major mode hook
+Each major mode should have a normal @dfn{mode hook} named
+@code{@var{modename}-mode-hook}.  The very last thing the major mode command
+should do is to call @code{run-mode-hooks}.  This runs the mode hook,
+and then runs the normal hook @code{after-change-major-mode-hook}.
+@xref{Mode Hooks}.
+
+@item
+The major mode command may start by calling some other major mode
+command (called the @dfn{parent mode}) and then alter some of its
+settings.  A mode that does this is called a @dfn{derived mode}.  The
+recommended way to define one is to use @code{define-derived-mode},
+but this is not required.  Such a mode should call the parent mode
+command inside a @code{delay-mode-hooks} form.  (Using
+@code{define-derived-mode} does this automatically.)  @xref{Derived
+Modes}, and @ref{Mode Hooks}.
+
+@item
+If something special should be done if the user switches a buffer from
+this mode to any other major mode, this mode can set up a buffer-local
+value for @code{change-major-mode-hook} (@pxref{Creating Buffer-Local}).
+
+@item
+If this mode is appropriate only for specially-prepared text, then the
+major mode command symbol should have a property named @code{mode-class}
+with value @code{special}, put on as follows:
+
+@kindex mode-class @r{(property)}
+@cindex @code{special}
+@example
+(put 'funny-mode 'mode-class 'special)
+@end example
+
+@noindent
+This tells Emacs that new buffers created while the current buffer is
+in Funny mode should not inherit Funny mode, in case
+@code{default-major-mode} is @code{nil}.  Modes such as Dired, Rmail,
+and Buffer List use this feature.
+
+@item
+If you want to make the new mode the default for files with certain
+recognizable names, add an element to @code{auto-mode-alist} to select
+the mode for those file names (@pxref{Auto Major Mode}).  If you
+define the mode command to autoload, you should add this element in
+the same file that calls @code{autoload}.  If you use an autoload
+cookie for the mode command, you can also use an autoload cookie for
+the form that adds the element (@pxref{autoload cookie}).  If you do
+not autoload the mode command, it is sufficient to add the element in
+the file that contains the mode definition.
+
+@item
+In the comments that document the file, you should provide a sample
+@code{autoload} form and an example of how to add to
+@code{auto-mode-alist}, that users can include in their init files
+(@pxref{Init File}).
+
+@item
+@cindex mode loading
+The top-level forms in the file defining the mode should be written so
+that they may be evaluated more than once without adverse consequences.
+Even if you never load the file more than once, someone else will.
+@end itemize
+
+@node Auto Major Mode
+@subsection How Emacs Chooses a Major Mode
+@cindex major mode, automatic selection
+
+  Based on information in the file name or in the file itself, Emacs
+automatically selects a major mode for the new buffer when a file is
+visited.  It also processes local variables specified in the file text.
+
+@deffn Command fundamental-mode
+  Fundamental mode is a major mode that is not specialized for anything
+in particular.  Other major modes are defined in effect by comparison
+with this one---their definitions say what to change, starting from
+Fundamental mode.  The @code{fundamental-mode} function does @emph{not}
+run any mode hooks; you're not supposed to customize it.  (If you want Emacs
+to behave differently in Fundamental mode, change the @emph{global}
+state of Emacs.)
+@end deffn
+
+@deffn Command normal-mode &optional find-file
+This function establishes the proper major mode and buffer-local variable
+bindings for the current buffer.  First it calls @code{set-auto-mode}
+(see below), then it runs @code{hack-local-variables} to parse, and
+bind or evaluate as appropriate, the file's local variables
+(@pxref{File Local Variables}).
+
+If the @var{find-file} argument to @code{normal-mode} is non-@code{nil},
+@code{normal-mode} assumes that the @code{find-file} function is calling
+it.  In this case, it may process local variables in the @samp{-*-}
+line or at the end of the file.  The variable
+@code{enable-local-variables} controls whether to do so.  @xref{File
+Variables, , Local Variables in Files, emacs, The GNU Emacs Manual},
+for the syntax of the local variables section of a file.
+
+If you run @code{normal-mode} interactively, the argument
+@var{find-file} is normally @code{nil}.  In this case,
+@code{normal-mode} unconditionally processes any file local variables.
+
+If @code{normal-mode} processes the local variables list and this list
+specifies a major mode, that mode overrides any mode chosen by
+@code{set-auto-mode}.  If neither @code{set-auto-mode} nor
+@code{hack-local-variables} specify a major mode, the buffer stays in
+the major mode determined by @code{default-major-mode} (see below).
+
+@cindex file mode specification error
+@code{normal-mode} uses @code{condition-case} around the call to the
+major mode function, so errors are caught and reported as a @samp{File
+mode specification error},  followed by the original error message.
+@end deffn
+
+@defun set-auto-mode &optional keep-mode-if-same
+@cindex visited file mode
+  This function selects the major mode that is appropriate for the
+current buffer.  It bases its decision (in order of precedence) on
+the @w{@samp{-*-}} line, on the @w{@samp{#!}} line (using
+@code{interpreter-mode-alist}), on the text at the beginning of the
+buffer (using @code{magic-mode-alist}), and finally on the visited
+file name (using @code{auto-mode-alist}).  @xref{Choosing Modes, , How
+Major Modes are Chosen, emacs, The GNU Emacs Manual}.  However, this
+function does not look for the @samp{mode:} local variable near the
+end of a file; the @code{hack-local-variables} function does that.
+If @code{enable-local-variables} is @code{nil}, @code{set-auto-mode}
+does not check the @w{@samp{-*-}} line for a mode tag either.
+
+If @var{keep-mode-if-same} is non-@code{nil}, this function does not
+call the mode command if the buffer is already in the proper major
+mode.  For instance, @code{set-visited-file-name} sets this to
+@code{t} to avoid killing buffer local variables that the user may
+have set.
+@end defun
+
+@defopt default-major-mode
+This variable holds the default major mode for new buffers.  The
+standard value is @code{fundamental-mode}.
+
+If the value of @code{default-major-mode} is @code{nil}, Emacs uses
+the (previously) current buffer's major mode as the default major mode
+of a new buffer.  However, if that major mode symbol has a @code{mode-class}
+property with value @code{special}, then it is not used for new buffers;
+Fundamental mode is used instead.  The modes that have this property are
+those such as Dired and Rmail that are useful only with text that has
+been specially prepared.
+@end defopt
+
+@defun set-buffer-major-mode buffer
+This function sets the major mode of @var{buffer} to the value of
+@code{default-major-mode}; if that variable is @code{nil}, it uses the
+current buffer's major mode (if that is suitable).  As an exception,
+if @var{buffer}'s name is @samp{*scratch*}, it sets the mode to
+@code{initial-major-mode}.
+
+The low-level primitives for creating buffers do not use this function,
+but medium-level commands such as @code{switch-to-buffer} and
+@code{find-file-noselect} use it whenever they create buffers.
+@end defun
+
+@defopt initial-major-mode
+@cindex @samp{*scratch*}
+The value of this variable determines the major mode of the initial
+@samp{*scratch*} buffer.  The value should be a symbol that is a major
+mode command.  The default value is @code{lisp-interaction-mode}.
+@end defopt
+
+@defvar interpreter-mode-alist
+This variable specifies major modes to use for scripts that specify a
+command interpreter in a @samp{#!} line.  Its value is an alist with
+elements of the form @code{(@var{interpreter} . @var{mode})}; for
+example, @code{("perl" . perl-mode)} is one element present by
+default.  The element says to use mode @var{mode} if the file
+specifies an interpreter which matches @var{interpreter}.
+@end defvar
+
+@defvar magic-mode-alist
+This variable's value is an alist with elements of the form
+@code{(@var{regexp} .  @var{function})}, where @var{regexp} is a
+regular expression and @var{function} is a function or @code{nil}.
+After visiting a file, @code{set-auto-mode} calls @var{function} if
+the text at the beginning of the buffer matches @var{regexp} and
+@var{function} is non-@code{nil}; if @var{function} is @code{nil},
+@code{auto-mode-alist} gets to decide the mode.
+@end defvar
+
+@defvar magic-fallback-mode-alist
+This works like @code{magic-mode-alist}, except that it is handled
+only if @code{auto-mode-alist} does not specify a mode for this file.
+@end defvar
+
+@defvar auto-mode-alist
+This variable contains an association list of file name patterns
+(regular expressions) and corresponding major mode commands.  Usually,
+the file name patterns test for suffixes, such as @samp{.el} and
+@samp{.c}, but this need not be the case.  An ordinary element of the
+alist looks like @code{(@var{regexp} .  @var{mode-function})}.
+
+For example,
+
+@smallexample
+@group
+(("\\`/tmp/fol/" . text-mode)
+ ("\\.texinfo\\'" . texinfo-mode)
+ ("\\.texi\\'" . texinfo-mode)
+@end group
+@group
+ ("\\.el\\'" . emacs-lisp-mode)
+ ("\\.c\\'" . c-mode)
+ ("\\.h\\'" . c-mode)
+ @dots{})
+@end group
+@end smallexample
+
+When you visit a file whose expanded file name (@pxref{File Name
+Expansion}), with version numbers and backup suffixes removed using
+@code{file-name-sans-versions} (@pxref{File Name Components}), matches
+a @var{regexp}, @code{set-auto-mode} calls the corresponding
+@var{mode-function}.  This feature enables Emacs to select the proper
+major mode for most files.
+
+If an element of @code{auto-mode-alist} has the form @code{(@var{regexp}
+@var{function} t)}, then after calling @var{function}, Emacs searches
+@code{auto-mode-alist} again for a match against the portion of the file
+name that did not match before.  This feature is useful for
+uncompression packages: an entry of the form @code{("\\.gz\\'"
+@var{function} t)} can uncompress the file and then put the uncompressed
+file in the proper mode according to the name sans @samp{.gz}.
+
+Here is an example of how to prepend several pattern pairs to
+@code{auto-mode-alist}.  (You might use this sort of expression in your
+init file.)
+
+@smallexample
+@group
+(setq auto-mode-alist
+  (append
+   ;; @r{File name (within directory) starts with a dot.}
+   '(("/\\.[^/]*\\'" . fundamental-mode)
+     ;; @r{File name has no dot.}
+     ("[^\\./]*\\'" . fundamental-mode)
+     ;; @r{File name ends in @samp{.C}.}
+     ("\\.C\\'" . c++-mode))
+   auto-mode-alist))
+@end group
+@end smallexample
+@end defvar
+
+@node Mode Help
+@subsection Getting Help about a Major Mode
+@cindex mode help
+@cindex help for major mode
+@cindex documentation for major mode
+
+  The @code{describe-mode} function is used to provide information
+about major modes.  It is normally called with @kbd{C-h m}.  The
+@code{describe-mode} function uses the value of @code{major-mode},
+which is why every major mode function needs to set the
+@code{major-mode} variable.
+
+@deffn Command describe-mode
+This function displays the documentation of the current major mode.
+
+The @code{describe-mode} function calls the @code{documentation}
+function using the value of @code{major-mode} as an argument.  Thus, it
+displays the documentation string of the major mode function.
+(@xref{Accessing Documentation}.)
+@end deffn
+
+@defvar major-mode
+This buffer-local variable holds the symbol for the current buffer's
+major mode.  This symbol should have a function definition that is the
+command to switch to that major mode.  The @code{describe-mode}
+function uses the documentation string of the function as the
+documentation of the major mode.
+@end defvar
+
+@node Derived Modes
+@subsection Defining Derived Modes
+@cindex derived mode
+
+  It's often useful to define a new major mode in terms of an existing
+one.  An easy way to do this is to use @code{define-derived-mode}.
+
+@defmac define-derived-mode variant parent name docstring keyword-args@dots{} body@dots{}
+This construct defines @var{variant} as a major mode command, using
+@var{name} as the string form of the mode name.  @var{variant} and
+@var{parent} should be unquoted symbols.
+
+The new command @var{variant} is defined to call the function
+@var{parent}, then override certain aspects of that parent mode:
+
+@itemize @bullet
+@item
+The new mode has its own sparse keymap, named
+@code{@var{variant}-map}.  @code{define-derived-mode}
+makes the parent mode's keymap the parent of the new map, unless
+@code{@var{variant}-map} is already set and already has a parent.
+
+@item
+The new mode has its own syntax table, kept in the variable
+@code{@var{variant}-syntax-table}, unless you override this using the
+@code{:syntax-table} keyword (see below).  @code{define-derived-mode}
+makes the parent mode's syntax-table the parent of
+@code{@var{variant}-syntax-table}, unless the latter is already set
+and already has a parent different from the standard syntax table.
+
+@item
+The new mode has its own abbrev table, kept in the variable
+@code{@var{variant}-abbrev-table}, unless you override this using the
+@code{:abbrev-table} keyword (see below).
+
+@item
+The new mode has its own mode hook, @code{@var{variant}-hook}.  It
+runs this hook, after running the hooks of its ancestor modes, with
+@code{run-mode-hooks}, as the last thing it does. @xref{Mode Hooks}.
+@end itemize
+
+In addition, you can specify how to override other aspects of
+@var{parent} with @var{body}.  The command @var{variant}
+evaluates the forms in @var{body} after setting up all its usual
+overrides, just before running the mode hooks.
+
+You can also specify @code{nil} for @var{parent}.  This gives the new
+mode no parent.  Then @code{define-derived-mode} behaves as described
+above, but, of course, omits all actions connected with @var{parent}.
+
+The argument @var{docstring} specifies the documentation string for
+the new mode.  @code{define-derived-mode} adds some general
+information about the mode's hook, followed by the mode's keymap, at
+the end of this docstring.  If you omit @var{docstring},
+@code{define-derived-mode} generates a documentation string.
+
+The @var{keyword-args} are pairs of keywords and values.  The values
+are evaluated.  The following keywords are currently supported:
+
+@table @code
+@item :syntax-table
+You can use this to explicitly specify a syntax table for the new
+mode.  If you specify a @code{nil} value, the new mode uses the same
+syntax table as @var{parent}, or the standard syntax table if
+@var{parent} is @code{nil}.  (Note that this does @emph{not} follow
+the convention used for non-keyword arguments that a @code{nil} value
+is equivalent with not specifying the argument.)
+
+@item :abbrev-table
+You can use this to explicitly specify an abbrev table for the new
+mode.  If you specify a @code{nil} value, the new mode uses the same
+abbrev table as @var{parent}, or @code{fundamental-mode-abbrev-table}
+if @var{parent} is @code{nil}.  (Again, a @code{nil} value is
+@emph{not} equivalent to not specifying this keyword.)
+
+@item :group
+If this is specified, the value should be the customization group for
+this mode.  (Not all major modes have one.)  Only the (still
+experimental and unadvertised) command @code{customize-mode} currently
+uses this.  @code{define-derived-mode} does @emph{not} automatically
+define the specified customization group.
+@end table
+
+Here is a hypothetical example:
+
+@example
+(define-derived-mode hypertext-mode
+  text-mode "Hypertext"
+  "Major mode for hypertext.
+\\@{hypertext-mode-map@}"
+  (setq case-fold-search nil))
+
+(define-key hypertext-mode-map
+  [down-mouse-3] 'do-hyper-link)
+@end example
+
+Do not write an @code{interactive} spec in the definition;
+@code{define-derived-mode} does that automatically.
+@end defmac
+
+@node Generic Modes
+@subsection Generic Modes
+@cindex generic mode
+
+  @dfn{Generic modes} are simple major modes with basic support for
+comment syntax and Font Lock mode.  To define a generic mode, use the
+macro @code{define-generic-mode}.  See the file @file{generic-x.el}
+for some examples of the use of @code{define-generic-mode}.
+
+@defmac define-generic-mode mode comment-list keyword-list font-lock-list auto-mode-list function-list &optional docstring
+This macro defines a generic mode command named @var{mode} (a symbol,
+not quoted).  The optional argument @var{docstring} is the
+documentation for the mode command.  If you do not supply it,
+@code{define-generic-mode} generates one by default.
+
+The argument @var{comment-list} is a list in which each element is
+either a character, a string of one or two characters, or a cons cell.
+A character or a string is set up in the mode's syntax table as a
+``comment starter.''  If the entry is a cons cell, the @sc{car} is set
+up as a ``comment starter'' and the @sc{cdr} as a ``comment ender.''
+(Use @code{nil} for the latter if you want comments to end at the end
+of the line.)  Note that the syntax table mechanism has limitations
+about what comment starters and enders are actually possible.
+@xref{Syntax Tables}.
+
+The argument @var{keyword-list} is a list of keywords to highlight
+with @code{font-lock-keyword-face}.  Each keyword should be a string.
+Meanwhile, @var{font-lock-list} is a list of additional expressions to
+highlight.  Each element of this list should have the same form as an
+element of @code{font-lock-keywords}.  @xref{Search-based
+Fontification}.
+
+The argument @var{auto-mode-list} is a list of regular expressions to
+add to the variable @code{auto-mode-alist}.  They are added by the execution
+of the @code{define-generic-mode} form, not by expanding the macro call.
+
+Finally, @var{function-list} is a list of functions for the mode
+command to call for additional setup.  It calls these functions just
+before it runs the mode hook variable @code{@var{mode}-hook}.
+@end defmac
+
+@node Mode Hooks
+@subsection Mode Hooks
+
+  Every major mode function should finish by running its mode hook and
+the mode-independent normal hook @code{after-change-major-mode-hook}.
+It does this by calling @code{run-mode-hooks}.  If the major mode is a
+derived mode, that is if it calls another major mode (the parent mode)
+in its body, it should do this inside @code{delay-mode-hooks} so that
+the parent won't run these hooks itself.  Instead, the derived mode's
+call to @code{run-mode-hooks} runs the parent's mode hook too.
+@xref{Major Mode Conventions}.
+
+  Emacs versions before Emacs 22 did not have @code{delay-mode-hooks}.
+When user-implemented major modes have not been updated to use it,
+they won't entirely follow these conventions: they may run the
+parent's mode hook too early, or fail to run
+@code{after-change-major-mode-hook}.  If you encounter such a major
+mode, please correct it to follow these conventions.
+
+  When you defined a major mode using @code{define-derived-mode}, it
+automatically makes sure these conventions are followed.  If you
+define a major mode ``by hand,'' not using @code{define-derived-mode},
+use the following functions to handle these conventions automatically.
+
+@defun run-mode-hooks &rest hookvars
+Major modes should run their mode hook using this function.  It is
+similar to @code{run-hooks} (@pxref{Hooks}), but it also runs
+@code{after-change-major-mode-hook}.
+
+When this function is called during the execution of a
+@code{delay-mode-hooks} form, it does not run the hooks immediately.
+Instead, it arranges for the next call to @code{run-mode-hooks} to run
+them.
+@end defun
+
+@defmac delay-mode-hooks body@dots{}
+When one major mode command calls another, it should do so inside of
+@code{delay-mode-hooks}.
+
+This macro executes @var{body}, but tells all @code{run-mode-hooks}
+calls during the execution of @var{body} to delay running their hooks.
+The hooks will actually run during the next call to
+@code{run-mode-hooks} after the end of the @code{delay-mode-hooks}
+construct.
+@end defmac
+
+@defvar after-change-major-mode-hook
+This is a normal hook run by @code{run-mode-hooks}.  It is run at the
+very end of every properly-written major mode function.
+@end defvar
+
+@node Example Major Modes
+@subsection Major Mode Examples
+
+  Text mode is perhaps the simplest mode besides Fundamental mode.
+Here are excerpts from  @file{text-mode.el} that illustrate many of
+the conventions listed above:
+
+@smallexample
+@group
+;; @r{Create the syntax table for this mode.}
+(defvar text-mode-syntax-table
+  (let ((st (make-syntax-table)))
+    (modify-syntax-entry ?\" ".   " st)
+    (modify-syntax-entry ?\\ ".   " st)
+    ;; Add `p' so M-c on `hello' leads to `Hello', not `hello'.
+    (modify-syntax-entry ?' "w p" st)
+    st)
+  "Syntax table used while in `text-mode'.")
+@end group
+
+;; @r{Create the keymap for this mode.}
+@group
+(defvar text-mode-map
+  (let ((map (make-sparse-keymap)))
+    (define-key map "\e\t" 'ispell-complete-word)
+    (define-key map "\es" 'center-line)
+    (define-key map "\eS" 'center-paragraph)
+    map)
+  "Keymap for `text-mode'.
+Many other modes, such as Mail mode, Outline mode
+and Indented Text mode, inherit all the commands
+defined in this map.")
+@end group
+@end smallexample
+
+  Here is how the actual mode command is defined now:
+
+@smallexample
+@group
+(define-derived-mode text-mode nil "Text"
+  "Major mode for editing text written for humans to read.
+In this mode, paragraphs are delimited only by blank or white lines.
+You can thus get the full benefit of adaptive filling
+ (see the variable `adaptive-fill-mode').
+\\@{text-mode-map@}
+Turning on Text mode runs the normal hook `text-mode-hook'."
+@end group
+@group
+  (make-local-variable 'text-mode-variant)
+  (setq text-mode-variant t)
+  ;; @r{These two lines are a feature added recently.}
+  (set (make-local-variable 'require-final-newline)
+       mode-require-final-newline)
+  (set (make-local-variable 'indent-line-function) 'indent-relative))
+@end group
+@end smallexample
+
+@noindent
+(The last line is redundant nowadays, since @code{indent-relative} is
+the default value, and we'll delete it in a future version.)
+
+  Here is how it was defined formerly, before
+@code{define-derived-mode} existed:
+
+@smallexample
+@group
+;; @r{This isn't needed nowadays, since @code{define-derived-mode} does it.}
+(defvar text-mode-abbrev-table nil
+  "Abbrev table used while in text mode.")
+(define-abbrev-table 'text-mode-abbrev-table ())
+@end group
+
+@group
+(defun text-mode ()
+  "Major mode for editing text intended for humans to read...
+ Special commands: \\@{text-mode-map@}
+@end group
+@group
+Turning on text-mode runs the hook `text-mode-hook'."
+  (interactive)
+  (kill-all-local-variables)
+  (use-local-map text-mode-map)
+@end group
+@group
+  (setq local-abbrev-table text-mode-abbrev-table)
+  (set-syntax-table text-mode-syntax-table)
+@end group
+@group
+  ;; @r{These four lines are absent from the current version}
+  ;; @r{not because this is done some other way, but rather}
+  ;; @r{because nowadays Text mode uses the normal definition of paragraphs.}
+  (make-local-variable 'paragraph-start)
+  (setq paragraph-start (concat "[ \t]*$\\|" page-delimiter))
+  (make-local-variable 'paragraph-separate)
+  (setq paragraph-separate paragraph-start)
+  (make-local-variable 'indent-line-function)
+  (setq indent-line-function 'indent-relative-maybe)
+@end group
+@group
+  (setq mode-name "Text")
+  (setq major-mode 'text-mode)
+  (run-mode-hooks 'text-mode-hook)) ; @r{Finally, this permits the user to}
+                                    ;   @r{customize the mode with a hook.}
+@end group
+@end smallexample
+
+@cindex @file{lisp-mode.el}
+  The three Lisp modes (Lisp mode, Emacs Lisp mode, and Lisp
+Interaction mode) have more features than Text mode and the code is
+correspondingly more complicated.  Here are excerpts from
+@file{lisp-mode.el} that illustrate how these modes are written.
+
+@cindex syntax table example
+@smallexample
+@group
+;; @r{Create mode-specific table variables.}
+(defvar lisp-mode-syntax-table nil "")
+(defvar lisp-mode-abbrev-table nil "")
+@end group
+
+@group
+(defvar emacs-lisp-mode-syntax-table
+  (let ((table (make-syntax-table)))
+    (let ((i 0))
+@end group
+
+@group
+      ;; @r{Set syntax of chars up to @samp{0} to say they are}
+      ;;   @r{part of symbol names but not words.}
+      ;;   @r{(The digit @samp{0} is @code{48} in the @acronym{ASCII} character set.)}
+      (while (< i ?0)
+        (modify-syntax-entry i "_   " table)
+        (setq i (1+ i)))
+      ;; @r{@dots{} similar code follows for other character ranges.}
+@end group
+@group
+      ;; @r{Then set the syntax codes for characters that are special in Lisp.}
+      (modify-syntax-entry ?  "    " table)
+      (modify-syntax-entry ?\t "    " table)
+      (modify-syntax-entry ?\f "    " table)
+      (modify-syntax-entry ?\n ">   " table)
+@end group
+@group
+      ;; @r{Give CR the same syntax as newline, for selective-display.}
+      (modify-syntax-entry ?\^m ">   " table)
+      (modify-syntax-entry ?\; "<   " table)
+      (modify-syntax-entry ?` "'   " table)
+      (modify-syntax-entry ?' "'   " table)
+      (modify-syntax-entry ?, "'   " table)
+@end group
+@group
+      ;; @r{@dots{}likewise for many other characters@dots{}}
+      (modify-syntax-entry ?\( "()  " table)
+      (modify-syntax-entry ?\) ")(  " table)
+      (modify-syntax-entry ?\[ "(]  " table)
+      (modify-syntax-entry ?\] ")[  " table))
+    table))
+@end group
+@group
+;; @r{Create an abbrev table for lisp-mode.}
+(define-abbrev-table 'lisp-mode-abbrev-table ())
+@end group
+@end smallexample
+
+  The three modes for Lisp share much of their code.  For instance,
+each calls the following function to set various variables:
+
+@smallexample
+@group
+(defun lisp-mode-variables (lisp-syntax)
+  (when lisp-syntax
+    (set-syntax-table lisp-mode-syntax-table))
+  (setq local-abbrev-table lisp-mode-abbrev-table)
+  @dots{}
+@end group
+@end smallexample
+
+  In Lisp and most programming languages, we want the paragraph
+commands to treat only blank lines as paragraph separators.  And the
+modes should understand the Lisp conventions for comments.  The rest of
+@code{lisp-mode-variables} sets this up:
+
+@smallexample
+@group
+  (make-local-variable 'paragraph-start)
+  (setq paragraph-start (concat page-delimiter "\\|$" ))
+  (make-local-variable 'paragraph-separate)
+  (setq paragraph-separate paragraph-start)
+  @dots{}
+@end group
+@group
+  (make-local-variable 'comment-indent-function)
+  (setq comment-indent-function 'lisp-comment-indent))
+  @dots{}
+@end group
+@end smallexample
+
+  Each of the different Lisp modes has a slightly different keymap.  For
+example, Lisp mode binds @kbd{C-c C-z} to @code{run-lisp}, but the other
+Lisp modes do not.  However, all Lisp modes have some commands in
+common.  The following code sets up the common commands:
+
+@smallexample
+@group
+(defvar shared-lisp-mode-map ()
+  "Keymap for commands shared by all sorts of Lisp modes.")
+
+;; @r{Putting this @code{if} after the @code{defvar} is an older style.}
+(if shared-lisp-mode-map
+    ()
+   (setq shared-lisp-mode-map (make-sparse-keymap))
+   (define-key shared-lisp-mode-map "\e\C-q" 'indent-sexp)
+   (define-key shared-lisp-mode-map "\177"
+               'backward-delete-char-untabify))
+@end group
+@end smallexample
+
+@noindent
+And here is the code to set up the keymap for Lisp mode:
+
+@smallexample
+@group
+(defvar lisp-mode-map ()
+  "Keymap for ordinary Lisp mode...")
+
+(if lisp-mode-map
+    ()
+  (setq lisp-mode-map (make-sparse-keymap))
+  (set-keymap-parent lisp-mode-map shared-lisp-mode-map)
+  (define-key lisp-mode-map "\e\C-x" 'lisp-eval-defun)
+  (define-key lisp-mode-map "\C-c\C-z" 'run-lisp))
+@end group
+@end smallexample
+
+  Finally, here is the complete major mode function definition for
+Lisp mode.
+
+@smallexample
+@group
+(defun lisp-mode ()
+  "Major mode for editing Lisp code for Lisps other than GNU Emacs Lisp.
+Commands:
+Delete converts tabs to spaces as it moves back.
+Blank lines separate paragraphs.  Semicolons start comments.
+\\@{lisp-mode-map@}
+Note that `run-lisp' may be used either to start an inferior Lisp job
+or to switch back to an existing one.
+@end group
+
+@group
+Entry to this mode calls the value of `lisp-mode-hook'
+if that value is non-nil."
+  (interactive)
+  (kill-all-local-variables)
+@end group
+@group
+  (use-local-map lisp-mode-map)          ; @r{Select the mode's keymap.}
+  (setq major-mode 'lisp-mode)           ; @r{This is how @code{describe-mode}}
+                                         ;   @r{finds out what to describe.}
+  (setq mode-name "Lisp")                ; @r{This goes into the mode line.}
+  (lisp-mode-variables t)                ; @r{This defines various variables.}
+  (make-local-variable 'comment-start-skip)
+  (setq comment-start-skip
+        "\\(\\(^\\|[^\\\\\n]\\)\\(\\\\\\\\\\)*\\)\\(;+\\|#|\\) *")
+  (make-local-variable 'font-lock-keywords-case-fold-search)
+  (setq font-lock-keywords-case-fold-search t)
+@end group
+@group
+  (setq imenu-case-fold-search t)
+  (set-syntax-table lisp-mode-syntax-table)
+  (run-mode-hooks 'lisp-mode-hook))      ; @r{This permits the user to use a}
+                                         ;   @r{hook to customize the mode.}
+@end group
+@end smallexample
+
+@node Minor Modes
+@section Minor Modes
+@cindex minor mode
+
+  A @dfn{minor mode} provides features that users may enable or disable
+independently of the choice of major mode.  Minor modes can be enabled
+individually or in combination.  Minor modes would be better named
+``generally available, optional feature modes,'' except that such a name
+would be unwieldy.
+
+  A minor mode is not usually meant as a variation of a single major mode.
+Usually they are general and can apply to many major modes.  For
+example, Auto Fill mode works with any major mode that permits text
+insertion.  To be general, a minor mode must be effectively independent
+of the things major modes do.
+
+  A minor mode is often much more difficult to implement than a major
+mode.  One reason is that you should be able to activate and deactivate
+minor modes in any order.  A minor mode should be able to have its
+desired effect regardless of the major mode and regardless of the other
+minor modes in effect.
+
+  Often the biggest problem in implementing a minor mode is finding a
+way to insert the necessary hook into the rest of Emacs.  Minor mode
+keymaps make this easier than it used to be.
+
+@defvar minor-mode-list
+The value of this variable is a list of all minor mode commands.
+@end defvar
+
+@menu
+* Minor Mode Conventions::      Tips for writing a minor mode.
+* Keymaps and Minor Modes::     How a minor mode can have its own keymap.
+* Defining Minor Modes::        A convenient facility for defining minor modes.
+@end menu
+
+@node Minor Mode Conventions
+@subsection Conventions for Writing Minor Modes
+@cindex minor mode conventions
+@cindex conventions for writing minor modes
+
+  There are conventions for writing minor modes just as there are for
+major modes.  Several of the major mode conventions apply to minor
+modes as well: those regarding the name of the mode initialization
+function, the names of global symbols, the use of a hook at the end of
+the initialization function, and the use of keymaps and other tables.
+
+  In addition, there are several conventions that are specific to
+minor modes.  (The easiest way to follow all the conventions is to use
+the macro @code{define-minor-mode}; @ref{Defining Minor Modes}.)
+
+@itemize @bullet
+@item
+@cindex mode variable
+Make a variable whose name ends in @samp{-mode} to control the minor
+mode.  We call this the @dfn{mode variable}.  The minor mode command
+should set this variable (@code{nil} to disable; anything else to
+enable).
+
+If possible, implement the mode so that setting the variable
+automatically enables or disables the mode.  Then the minor mode command
+does not need to do anything except set the variable.
+
+This variable is used in conjunction with the @code{minor-mode-alist} to
+display the minor mode name in the mode line.  It can also enable
+or disable a minor mode keymap.  Individual commands or hooks can also
+check the variable's value.
+
+If you want the minor mode to be enabled separately in each buffer,
+make the variable buffer-local.
+
+@item
+Define a command whose name is the same as the mode variable.
+Its job is to enable and disable the mode by setting the variable.
+
+The command should accept one optional argument.  If the argument is
+@code{nil}, it should toggle the mode (turn it on if it is off, and
+off if it is on).  It should turn the mode on if the argument is a
+positive integer, the symbol @code{t}, or a list whose @sc{car} is one
+of those.  It should turn the mode off if the argument is a negative
+integer or zero, the symbol @code{-}, or a list whose @sc{car} is a
+negative integer or zero.  The meaning of other arguments is not
+specified.
+
+Here is an example taken from the definition of @code{transient-mark-mode}.
+It shows the use of @code{transient-mark-mode} as a variable that enables or
+disables the mode's behavior, and also shows the proper way to toggle,
+enable or disable the minor mode based on the raw prefix argument value.
+
+@smallexample
+@group
+(setq transient-mark-mode
+      (if (null arg) (not transient-mark-mode)
+        (> (prefix-numeric-value arg) 0)))
+@end group
+@end smallexample
+
+@item
+Add an element to @code{minor-mode-alist} for each minor mode
+(@pxref{Definition of minor-mode-alist}), if you want to indicate the
+minor mode in the mode line.  This element should be a list of the
+following form:
+
+@smallexample
+(@var{mode-variable} @var{string})
+@end smallexample
+
+Here @var{mode-variable} is the variable that controls enabling of the
+minor mode, and @var{string} is a short string, starting with a space,
+to represent the mode in the mode line.  These strings must be short so
+that there is room for several of them at once.
+
+When you add an element to @code{minor-mode-alist}, use @code{assq} to
+check for an existing element, to avoid duplication.  For example:
+
+@smallexample
+@group
+(unless (assq 'leif-mode minor-mode-alist)
+  (setq minor-mode-alist
+        (cons '(leif-mode " Leif") minor-mode-alist)))
+@end group
+@end smallexample
+
+@noindent
+or like this, using @code{add-to-list} (@pxref{List Variables}):
+
+@smallexample
+@group
+(add-to-list 'minor-mode-alist '(leif-mode " Leif"))
+@end group
+@end smallexample
+@end itemize
+
+  Global minor modes distributed with Emacs should if possible support
+enabling and disabling via Custom (@pxref{Customization}).  To do this,
+the first step is to define the mode variable with @code{defcustom}, and
+specify @code{:type boolean}.
+
+  If just setting the variable is not sufficient to enable the mode, you
+should also specify a @code{:set} method which enables the mode by
+invoking the mode command.  Note in the variable's documentation string that
+setting the variable other than via Custom may not take effect.
+
+  Also mark the definition with an autoload cookie (@pxref{autoload cookie}),
+and specify a @code{:require} so that customizing the variable will load
+the library that defines the mode.  This will copy suitable definitions
+into @file{loaddefs.el} so that users can use @code{customize-option} to
+enable the mode.  For example:
+
+@smallexample
+@group
+
+;;;###autoload
+(defcustom msb-mode nil
+  "Toggle msb-mode.
+Setting this variable directly does not take effect;
+use either \\[customize] or the function `msb-mode'."
+  :set 'custom-set-minor-mode
+  :initialize 'custom-initialize-default
+  :version "20.4"
+  :type    'boolean
+  :group   'msb
+  :require 'msb)
+@end group
+@end smallexample
+
+@node Keymaps and Minor Modes
+@subsection Keymaps and Minor Modes
+
+  Each minor mode can have its own keymap, which is active when the mode
+is enabled.  To set up a keymap for a minor mode, add an element to the
+alist @code{minor-mode-map-alist}.  @xref{Definition of minor-mode-map-alist}.
+
+@cindex @code{self-insert-command}, minor modes
+  One use of minor mode keymaps is to modify the behavior of certain
+self-inserting characters so that they do something else as well as
+self-insert.  In general, this is the only way to do that, since the
+facilities for customizing @code{self-insert-command} are limited to
+special cases (designed for abbrevs and Auto Fill mode).  (Do not try
+substituting your own definition of @code{self-insert-command} for the
+standard one.  The editor command loop handles this function specially.)
+
+The key sequences bound in a minor mode should consist of @kbd{C-c}
+followed by one of @kbd{.,/?`'"[]\|~!#$%^&*()-_+=}.  (The other
+punctuation characters are reserved for major modes.)
+
+@node Defining Minor Modes
+@subsection Defining Minor Modes
+
+  The macro @code{define-minor-mode} offers a convenient way of
+implementing a mode in one self-contained definition.
+
+@defmac define-minor-mode mode doc [init-value [lighter [keymap]]] keyword-args@dots{} body@dots{}
+This macro defines a new minor mode whose name is @var{mode} (a
+symbol).  It defines a command named @var{mode} to toggle the minor
+mode, with @var{doc} as its documentation string.  It also defines a
+variable named @var{mode}, which is set to @code{t} or @code{nil} by
+enabling or disabling the mode.  The variable is initialized to
+@var{init-value}.  Except in unusual circumstances (see below), this
+value must be @code{nil}.
+
+The string @var{lighter} says what to display in the mode line
+when the mode is enabled; if it is @code{nil}, the mode is not displayed
+in the mode line.
+
+The optional argument @var{keymap} specifies the keymap for the minor mode.
+It can be a variable name, whose value is the keymap, or it can be an alist
+specifying bindings in this form:
+
+@example
+(@var{key-sequence} . @var{definition})
+@end example
+
+The above three arguments @var{init-value}, @var{lighter}, and
+@var{keymap} can be (partially) omitted when @var{keyword-args} are
+used.  The @var{keyword-args} consist of keywords followed by
+corresponding values.  A few keywords have special meanings:
+
+@table @code
+@item :group @var{group}
+Custom group name to use in all generated @code{defcustom} forms.
+Defaults to @var{mode} without the possible trailing @samp{-mode}.
+@strong{Warning:} don't use this default group name unless you have
+written a @code{defgroup} to define that group properly.  @xref{Group
+Definitions}.
+
+@item :global @var{global}
+If non-@code{nil}, this specifies that the minor mode should be global
+rather than buffer-local.  It defaults to @code{nil}.
+
+One of the effects of making a minor mode global is that the
+@var{mode} variable becomes a customization variable.  Toggling it
+through the Custom interface turns the mode on and off, and its value
+can be saved for future Emacs sessions (@pxref{Saving
+Customizations,,, emacs, The GNU Emacs Manual}.  For the saved
+variable to work, you should ensure that the @code{define-minor-mode}
+form is evaluated each time Emacs starts; for packages that are not
+part of Emacs, the easiest way to do this is to specify a
+@code{:require} keyword.
+
+@item :init-value @var{init-value}
+This is equivalent to specifying @var{init-value} positionally.
+
+@item :lighter @var{lighter}
+This is equivalent to specifying @var{lighter} positionally.
+
+@item :keymap @var{keymap}
+This is equivalent to specifying @var{keymap} positionally.
+@end table
+
+Any other keyword arguments are passed directly to the
+@code{defcustom} generated for the variable @var{mode}.
+
+The command named @var{mode} first performs the standard actions such
+as setting the variable named @var{mode} and then executes the
+@var{body} forms, if any.  It finishes by running the mode hook
+variable @code{@var{mode}-hook}.
+@end defmac
+
+  The initial value must be @code{nil} except in cases where (1) the
+mode is preloaded in Emacs, or (2) it is painless for loading to
+enable the mode even though the user did not request it.  For
+instance, if the mode has no effect unless something else is enabled,
+and will always be loaded by that time, enabling it by default is
+harmless.  But these are unusual circumstances.  Normally, the
+initial value must be @code{nil}.
+
+@findex easy-mmode-define-minor-mode
+  The name @code{easy-mmode-define-minor-mode} is an alias
+for this macro.
+
+  Here is an example of using @code{define-minor-mode}:
+
+@smallexample
+(define-minor-mode hungry-mode
+  "Toggle Hungry mode.
+With no argument, this command toggles the mode.
+Non-null prefix argument turns on the mode.
+Null prefix argument turns off the mode.
+
+When Hungry mode is enabled, the control delete key
+gobbles all preceding whitespace except the last.
+See the command \\[hungry-electric-delete]."
+ ;; The initial value.
+ nil
+ ;; The indicator for the mode line.
+ " Hungry"
+ ;; The minor mode bindings.
+ '(("\C-\^?" . hungry-electric-delete))
+ :group 'hunger)
+@end smallexample
+
+@noindent
+This defines a minor mode named ``Hungry mode,'' a command named
+@code{hungry-mode} to toggle it, a variable named @code{hungry-mode}
+which indicates whether the mode is enabled, and a variable named
+@code{hungry-mode-map} which holds the keymap that is active when the
+mode is enabled.  It initializes the keymap with a key binding for
+@kbd{C-@key{DEL}}.  It puts the variable @code{hungry-mode} into
+custom group @code{hunger}.  There are no @var{body} forms---many
+minor modes don't need any.
+
+  Here's an equivalent way to write it:
+
+@smallexample
+(define-minor-mode hungry-mode
+  "Toggle Hungry mode.
+With no argument, this command toggles the mode.
+Non-null prefix argument turns on the mode.
+Null prefix argument turns off the mode.
+
+When Hungry mode is enabled, the control delete key
+gobbles all preceding whitespace except the last.
+See the command \\[hungry-electric-delete]."
+ ;; The initial value.
+ :init-value nil
+ ;; The indicator for the mode line.
+ :lighter " Hungry"
+ ;; The minor mode bindings.
+ :keymap
+ '(("\C-\^?" . hungry-electric-delete)
+   ("\C-\M-\^?"
+    . (lambda ()
+        (interactive)
+        (hungry-electric-delete t))))
+ :group 'hunger)
+@end smallexample
+
+@defmac define-globalized-minor-mode global-mode mode turn-on keyword-args@dots{}
+This defines a global toggle named @var{global-mode} whose meaning is
+to enable or disable the buffer-local minor mode @var{mode} in all
+buffers.  To turn on the minor mode in a buffer, it uses the function
+@var{turn-on}; to turn off the minor mode, it calls @code{mode} with
+@minus{}1 as argument.
+
+Globally enabling the mode also affects buffers subsequently created
+by visiting files, and buffers that use a major mode other than
+Fundamental mode; but it does not detect the creation of a new buffer
+in Fundamental mode.
+
+This defines the customization option @var{global-mode} (@pxref{Customization}),
+which can be toggled in the Custom interface to turn the minor mode on
+and off.  As with @code{define-minor-mode}, you should ensure that the
+@code{define-globalized-minor-mode} form is evaluated each time Emacs
+starts, for example by providing a @code{:require} keyword.
+
+Use @code{:group @var{group}} in @var{keyword-args} to specify the
+custom group for the mode variable of the global minor mode.
+@end defmac
+
+@node Mode Line Format
+@section Mode-Line Format
+@cindex mode line
+
+  Each Emacs window (aside from minibuffer windows) typically has a mode
+line at the bottom, which displays status information about the buffer
+displayed in the window.  The mode line contains information about the
+buffer, such as its name, associated file, depth of recursive editing,
+and major and minor modes.  A window can also have a @dfn{header
+line}, which is much like the mode line but appears at the top of the
+window.
+
+  This section describes how to control the contents of the mode line
+and header line.  We include it in this chapter because much of the
+information displayed in the mode line relates to the enabled major and
+minor modes.
+
+@menu
+* Base: Mode Line Basics. Basic ideas of mode line control.
+* Data: Mode Line Data.   The data structure that controls the mode line.
+* Top: Mode Line Top.     The top level variable, mode-line-format.
+* Mode Line Variables::   Variables used in that data structure.
+* %-Constructs::          Putting information into a mode line.
+* Properties in Mode::    Using text properties in the mode line.
+* Header Lines::          Like a mode line, but at the top.
+* Emulating Mode Line::   Formatting text as the mode line would.
+@end menu
+
+@node Mode Line Basics
+@subsection Mode Line Basics
+
+  @code{mode-line-format} is a buffer-local variable that holds a
+@dfn{mode line construct}, a kind of template, which controls what is
+displayed on the mode line of the current buffer.  The value of
+@code{header-line-format} specifies the buffer's header line in the
+same way.  All windows for the same buffer use the same
+@code{mode-line-format} and @code{header-line-format}.
+
+  For efficiency, Emacs does not continuously recompute the mode
+line and header line of a window.  It does so when circumstances
+appear to call for it---for instance, if you change the window
+configuration, switch buffers, narrow or widen the buffer, scroll, or
+change the buffer's modification status.  If you modify any of the
+variables referenced by @code{mode-line-format} (@pxref{Mode Line
+Variables}), or any other variables and data structures that affect
+how text is displayed (@pxref{Display}), you may want to force an
+update of the mode line so as to display the new information or
+display it in the new way.
+
+@defun force-mode-line-update &optional all
+Force redisplay of the current buffer's mode line and header line.
+The next redisplay will update the mode line and header line based on
+the latest values of all relevant variables.  With optional
+non-@code{nil} @var{all}, force redisplay of all mode lines and header
+lines.
+
+This function also forces recomputation of the menu bar menus
+and the frame title.
+@end defun
+
+  The selected window's mode line is usually displayed in a different
+color using the face @code{mode-line}.  Other windows' mode lines
+appear in the face @code{mode-line-inactive} instead.  @xref{Faces}.
+
+@node Mode Line Data
+@subsection The Data Structure of the Mode Line
+@cindex mode-line construct
+
+  The mode-line contents are controlled by a data structure called a
+@dfn{mode-line construct}, made up of lists, strings, symbols, and
+numbers kept in buffer-local variables.  Each data type has a specific
+meaning for the mode-line appearance, as described below.  The same
+data structure is used for constructing frame titles (@pxref{Frame
+Titles}) and header lines (@pxref{Header Lines}).
+
+  A mode-line construct may be as simple as a fixed string of text,
+but it usually specifies how to combine fixed strings with variables'
+values to construct the text.  Many of these variables are themselves
+defined to have mode-line constructs as their values.
+
+  Here are the meanings of various data types as mode-line constructs:
+
+@table @code
+@cindex percent symbol in mode line
+@item @var{string}
+A string as a mode-line construct appears verbatim except for
+@dfn{@code{%}-constructs} in it.  These stand for substitution of
+other data; see @ref{%-Constructs}.
+
+If parts of the string have @code{face} properties, they control
+display of the text just as they would text in the buffer.  Any
+characters which have no @code{face} properties are displayed, by
+default, in the face @code{mode-line} or @code{mode-line-inactive}
+(@pxref{Standard Faces,,, emacs, The GNU Emacs Manual}).  The
+@code{help-echo} and @code{local-map} properties in @var{string} have
+special meanings.  @xref{Properties in Mode}.
+
+@item @var{symbol}
+A symbol as a mode-line construct stands for its value.  The value of
+@var{symbol} is used as a mode-line construct, in place of @var{symbol}.
+However, the symbols @code{t} and @code{nil} are ignored, as is any
+symbol whose value is void.
+
+There is one exception: if the value of @var{symbol} is a string, it is
+displayed verbatim: the @code{%}-constructs are not recognized.
+
+Unless @var{symbol} is marked as ``risky'' (i.e., it has a
+non-@code{nil} @code{risky-local-variable} property), all text
+properties specified in @var{symbol}'s value are ignored.  This
+includes the text properties of strings in @var{symbol}'s value, as
+well as all @code{:eval} and @code{:propertize} forms in it.  (The
+reason for this is security: non-risky variables could be set
+automatically from file variables without prompting the user.)
+
+@item (@var{string} @var{rest}@dots{})
+@itemx (@var{list} @var{rest}@dots{})
+A list whose first element is a string or list means to process all the
+elements recursively and concatenate the results.  This is the most
+common form of mode-line construct.
+
+@item (:eval @var{form})
+A list whose first element is the symbol @code{:eval} says to evaluate
+@var{form}, and use the result as a string to display.  Make sure this
+evaluation cannot load any files, as doing so could cause infinite
+recursion.
+
+@item (:propertize @var{elt} @var{props}@dots{})
+A list whose first element is the symbol @code{:propertize} says to
+process the mode-line construct @var{elt} recursively, then add the text
+properties specified by @var{props} to the result.  The argument
+@var{props} should consist of zero or more pairs @var{text-property}
+@var{value}.  (This feature is new as of Emacs 22.1.)
+
+@item (@var{symbol} @var{then} @var{else})
+A list whose first element is a symbol that is not a keyword specifies
+a conditional.  Its meaning depends on the value of @var{symbol}.  If
+@var{symbol} has a non-@code{nil} value, the second element,
+@var{then}, is processed recursively as a mode-line element.
+Otherwise, the third element, @var{else}, is processed recursively.
+You may omit @var{else}; then the mode-line element displays nothing
+if the value of @var{symbol} is @code{nil} or void.
+
+@item (@var{width} @var{rest}@dots{})
+A list whose first element is an integer specifies truncation or
+padding of the results of @var{rest}.  The remaining elements
+@var{rest} are processed recursively as mode-line constructs and
+concatenated together.  When @var{width} is positive, the result is
+space filled on the right if its width is less than @var{width}.  When
+@var{width} is negative, the result is truncated on the right to
+@minus{}@var{width} columns if its width exceeds @minus{}@var{width}.
+
+For example, the usual way to show what percentage of a buffer is above
+the top of the window is to use a list like this: @code{(-3 "%p")}.
+@end table
+
+@node Mode Line Top
+@subsection The Top Level of Mode Line Control
+
+  The variable in overall control of the mode line is
+@code{mode-line-format}.
+
+@defvar mode-line-format
+The value of this variable is a mode-line construct that controls the
+contents of the mode-line.  It is always buffer-local in all buffers.
+
+If you set this variable to @code{nil} in a buffer, that buffer does
+not have a mode line.  (A window that is just one line tall never
+displays a mode line.)
+@end defvar
+
+  The default value of @code{mode-line-format} is designed to use the
+values of other variables such as @code{mode-line-position} and
+@code{mode-line-modes} (which in turn incorporates the values of the
+variables @code{mode-name} and @code{minor-mode-alist}).  Very few
+modes need to alter @code{mode-line-format} itself.  For most
+purposes, it is sufficient to alter some of the variables that
+@code{mode-line-format} either directly or indirectly refers to.
+
+  If you do alter @code{mode-line-format} itself, the new value should
+use the same variables that appear in the default value (@pxref{Mode
+Line Variables}), rather than duplicating their contents or displaying
+the information in another fashion.  This way, customizations made by
+the user or by Lisp programs (such as @code{display-time} and major
+modes) via changes to those variables remain effective.
+
+  Here is an example of a @code{mode-line-format} that might be
+useful for @code{shell-mode}, since it contains the host name and default
+directory.
+
+@example
+@group
+(setq mode-line-format
+  (list "-"
+   'mode-line-mule-info
+   'mode-line-modified
+   'mode-line-frame-identification
+   "%b--"
+@end group
+@group
+   ;; @r{Note that this is evaluated while making the list.}
+   ;; @r{It makes a mode-line construct which is just a string.}
+   (getenv "HOST")
+@end group
+   ":"
+   'default-directory
+   "   "
+   'global-mode-string
+   "   %[("
+   '(:eval (mode-line-mode-name))
+   'mode-line-process
+   'minor-mode-alist
+   "%n"
+   ")%]--"
+@group
+   '(which-func-mode ("" which-func-format "--"))
+   '(line-number-mode "L%l--")
+   '(column-number-mode "C%c--")
+   '(-3 "%p")
+   "-%-"))
+@end group
+@end example
+
+@noindent
+(The variables @code{line-number-mode}, @code{column-number-mode}
+and @code{which-func-mode} enable particular minor modes; as usual,
+these variable names are also the minor mode command names.)
+
+@node Mode Line Variables
+@subsection Variables Used in the Mode Line
+
+  This section describes variables incorporated by the standard value
+of @code{mode-line-format} into the text of the mode line.  There is
+nothing inherently special about these variables; any other variables
+could have the same effects on the mode line if
+@code{mode-line-format}'s value were changed to use them.  However,
+various parts of Emacs set these variables on the understanding that
+they will control parts of the mode line; therefore, practically
+speaking, it is essential for the mode line to use them.
+
+@defvar mode-line-mule-info
+This variable holds the value of the mode-line construct that displays
+information about the language environment, buffer coding system, and
+current input method.  @xref{Non-ASCII Characters}.
+@end defvar
+
+@defvar mode-line-modified
+This variable holds the value of the mode-line construct that displays
+whether the current buffer is modified.
+
+The default value of @code{mode-line-modified} is @code{("%1*%1+")}.
+This means that the mode line displays @samp{**} if the buffer is
+modified, @samp{--} if the buffer is not modified, @samp{%%} if the
+buffer is read only, and @samp{%*} if the buffer is read only and
+modified.
+
+Changing this variable does not force an update of the mode line.
+@end defvar
+
+@defvar mode-line-frame-identification
+This variable identifies the current frame.  The default value is
+@code{"  "} if you are using a window system which can show multiple
+frames, or @code{"-%F  "} on an ordinary terminal which shows only one
+frame at a time.
+@end defvar
+
+@defvar mode-line-buffer-identification
+This variable identifies the buffer being displayed in the window.  Its
+default value is @code{("%12b")}, which displays the buffer name, padded
+with spaces to at least 12 columns.
+@end defvar
+
+@defvar mode-line-position
+This variable indicates the position in the buffer.  Here is a
+simplified version of its default value.  The actual default value
+also specifies addition of the @code{help-echo} text property.
+
+@example
+@group
+((-3 "%p")
+ (size-indication-mode (8 " of %I"))
+@end group
+@group
+ (line-number-mode
+  ((column-number-mode
+    (10 " (%l,%c)")
+    (6 " L%l")))
+  ((column-number-mode
+    (5 " C%c")))))
+@end group
+@end example
+
+This means that @code{mode-line-position} displays at least the buffer
+percentage and possibly the buffer size, the line number and the column
+number.
+@end defvar
+
+@defvar vc-mode
+The variable @code{vc-mode}, buffer-local in each buffer, records
+whether the buffer's visited file is maintained with version control,
+and, if so, which kind.  Its value is a string that appears in the mode
+line, or @code{nil} for no version control.
+@end defvar
+
+@defvar mode-line-modes
+This variable displays the buffer's major and minor modes.  Here is a
+simplified version of its default value.  The real default value also
+specifies addition of text properties.
+
+@example
+@group
+("%[(" mode-name
+ mode-line-process minor-mode-alist
+ "%n" ")%]--")
+@end group
+@end example
+
+So @code{mode-line-modes} normally also displays the recursive editing
+level, information on the process status and whether narrowing is in
+effect.
+@end defvar
+
+  The following three variables are used in @code{mode-line-modes}:
+
+@defvar mode-name
+This buffer-local variable holds the ``pretty'' name of the current
+buffer's major mode.  Each major mode should set this variable so that the
+mode name will appear in the mode line.
+@end defvar
+
+@defvar mode-line-process
+This buffer-local variable contains the mode-line information on process
+status in modes used for communicating with subprocesses.  It is
+displayed immediately following the major mode name, with no intervening
+space.  For example, its value in the @samp{*shell*} buffer is
+@code{(":%s")}, which allows the shell to display its status along
+with the major mode as: @samp{(Shell:run)}.  Normally this variable
+is @code{nil}.
+@end defvar
+
+@defvar minor-mode-alist
+@anchor{Definition of minor-mode-alist}
+This variable holds an association list whose elements specify how the
+mode line should indicate that a minor mode is active.  Each element of
+the @code{minor-mode-alist} should be a two-element list:
+
+@example
+(@var{minor-mode-variable} @var{mode-line-string})
+@end example
+
+More generally, @var{mode-line-string} can be any mode-line spec.  It
+appears in the mode line when the value of @var{minor-mode-variable}
+is non-@code{nil}, and not otherwise.  These strings should begin with
+spaces so that they don't run together.  Conventionally, the
+@var{minor-mode-variable} for a specific mode is set to a
+non-@code{nil} value when that minor mode is activated.
+
+@code{minor-mode-alist} itself is not buffer-local.  Each variable
+mentioned in the alist should be buffer-local if its minor mode can be
+enabled separately in each buffer.
+@end defvar
+
+@defvar global-mode-string
+This variable holds a mode-line spec that, by default, appears in the
+mode line just after the @code{which-func-mode} minor mode if set,
+else after @code{mode-line-modes}.  The command @code{display-time}
+sets @code{global-mode-string} to refer to the variable
+@code{display-time-string}, which holds a string containing the time
+and load information.
+
+The @samp{%M} construct substitutes the value of
+@code{global-mode-string}, but that is obsolete, since the variable is
+included in the mode line from @code{mode-line-format}.
+@end defvar
+
+  The variable @code{default-mode-line-format} is where
+@code{mode-line-format} usually gets its value:
+
+@defvar default-mode-line-format
+This variable holds the default @code{mode-line-format} for buffers
+that do not override it.  This is the same as @code{(default-value
+'mode-line-format)}.
+
+Here is a simplified version of the default value of
+@code{default-mode-line-format}.  The real default value also
+specifies addition of text properties.
+
+@example
+@group
+("-"
+ mode-line-mule-info
+ mode-line-modified
+ mode-line-frame-identification
+ mode-line-buffer-identification
+@end group
+ "   "
+ mode-line-position
+ (vc-mode vc-mode)
+ "   "
+@group
+ mode-line-modes
+ (which-func-mode ("" which-func-format "--"))
+ (global-mode-string ("--" global-mode-string))
+ "-%-")
+@end group
+@end example
+@end defvar
+
+@node %-Constructs
+@subsection @code{%}-Constructs in the Mode Line
+
+  Strings used as mode-line constructs can use certain
+@code{%}-constructs to substitute various kinds of data.  Here is a
+list of the defined @code{%}-constructs, and what they mean.  In any
+construct except @samp{%%}, you can add a decimal integer after the
+@samp{%} to specify a minimum field width.  If the width is less, the
+field is padded with spaces to the right.
+
+@table @code
+@item %b
+The current buffer name, obtained with the @code{buffer-name} function.
+@xref{Buffer Names}.
+
+@item %c
+The current column number of point.
+
+@item %e
+When Emacs is nearly out of memory for Lisp objects, a brief message
+saying so.  Otherwise, this is empty.
+
+@item %f
+The visited file name, obtained with the @code{buffer-file-name}
+function.  @xref{Buffer File Name}.
+
+@item %F
+The title (only on a window system) or the name of the selected frame.
+@xref{Basic Parameters}.
+
+@item %i
+The size of the accessible part of the current buffer; basically
+@code{(- (point-max) (point-min))}.
+
+@item %I
+Like @samp{%i}, but the size is printed in a more readable way by using
+@samp{k} for 10^3, @samp{M} for 10^6, @samp{G} for 10^9, etc., to
+abbreviate.
+
+@item %l
+The current line number of point, counting within the accessible portion
+of the buffer.
+
+@item %n
+@samp{Narrow} when narrowing is in effect; nothing otherwise (see
+@code{narrow-to-region} in @ref{Narrowing}).
+
+@item %p
+The percentage of the buffer text above the @strong{top} of window, or
+@samp{Top}, @samp{Bottom} or @samp{All}.  Note that the default
+mode-line specification truncates this to three characters.
+
+@item %P
+The percentage of the buffer text that is above the @strong{bottom} of
+the window (which includes the text visible in the window, as well as
+the text above the top), plus @samp{Top} if the top of the buffer is
+visible on screen; or @samp{Bottom} or @samp{All}.
+
+@item %s
+The status of the subprocess belonging to the current buffer, obtained with
+@code{process-status}.  @xref{Process Information}.
+
+@item %t
+Whether the visited file is a text file or a binary file.  This is a
+meaningful distinction only on certain operating systems (@pxref{MS-DOS
+File Types}).
+
+@item %z
+The mnemonics of keyboard, terminal, and buffer coding systems.
+
+@item %Z
+Like @samp{%z}, but including the end-of-line format.
+
+@item %*
+@samp{%} if the buffer is read only (see @code{buffer-read-only}); @*
+@samp{*} if the buffer is modified (see @code{buffer-modified-p}); @*
+@samp{-} otherwise.  @xref{Buffer Modification}.
+
+@item %+
+@samp{*} if the buffer is modified (see @code{buffer-modified-p}); @*
+@samp{%} if the buffer is read only (see @code{buffer-read-only}); @*
+@samp{-} otherwise.  This differs from @samp{%*} only for a modified
+read-only buffer.  @xref{Buffer Modification}.
+
+@item %&
+@samp{*} if the buffer is modified, and @samp{-} otherwise.
+
+@item %[
+An indication of the depth of recursive editing levels (not counting
+minibuffer levels): one @samp{[} for each editing level.
+@xref{Recursive Editing}.
+
+@item %]
+One @samp{]} for each recursive editing level (not counting minibuffer
+levels).
+
+@item %-
+Dashes sufficient to fill the remainder of the mode line.
+
+@item %%
+The character @samp{%}---this is how to include a literal @samp{%} in a
+string in which @code{%}-constructs are allowed.
+@end table
+
+The following two @code{%}-constructs are still supported, but they are
+obsolete, since you can get the same results with the variables
+@code{mode-name} and @code{global-mode-string}.
+
+@table @code
+@item %m
+The value of @code{mode-name}.
+
+@item %M
+The value of @code{global-mode-string}.
+@end table
+
+@node Properties in Mode
+@subsection Properties in the Mode Line
+@cindex text properties in the mode line
+
+  Certain text properties are meaningful in the
+mode line.  The @code{face} property affects the appearance of text; the
+@code{help-echo} property associates help strings with the text, and
+@code{local-map} can make the text mouse-sensitive.
+
+  There are four ways to specify text properties for text in the mode
+line:
+
+@enumerate
+@item
+Put a string with a text property directly into the mode-line data
+structure.
+
+@item
+Put a text property on a mode-line %-construct such as @samp{%12b}; then
+the expansion of the %-construct will have that same text property.
+
+@item
+Use a @code{(:propertize @var{elt} @var{props}@dots{})} construct to
+give @var{elt} a text property specified by @var{props}.
+
+@item
+Use a list containing @code{:eval @var{form}} in the mode-line data
+structure, and make @var{form} evaluate to a string that has a text
+property.
+@end enumerate
+
+  You can use the @code{local-map} property to specify a keymap.  This
+keymap only takes real effect for mouse clicks; binding character keys
+and function keys to it has no effect, since it is impossible to move
+point into the mode line.
+
+  When the mode line refers to a variable which does not have a
+non-@code{nil} @code{risky-local-variable} property, any text
+properties given or specified within that variable's values are
+ignored.  This is because such properties could otherwise specify
+functions to be called, and those functions could come from file
+local variables.
+
+@node Header Lines
+@subsection Window Header Lines
+@cindex header line (of a window)
+@cindex window header line
+
+  A window can have a @dfn{header line} at the
+top, just as it can have a mode line at the bottom.  The header line
+feature works just like the mode-line feature, except that it's
+controlled by different variables.
+
+@defvar header-line-format
+This variable, local in every buffer, specifies how to display the
+header line, for windows displaying the buffer.  The format of the value
+is the same as for @code{mode-line-format} (@pxref{Mode Line Data}).
+@end defvar
+
+@defvar default-header-line-format
+This variable holds the default @code{header-line-format} for buffers
+that do not override it.  This is the same as @code{(default-value
+'header-line-format)}.
+
+It is normally @code{nil}, so that ordinary buffers have no header line.
+@end defvar
+
+  A window that is just one line tall never displays a header line.  A
+window that is two lines tall cannot display both a mode line and a
+header line at once; if it has a mode line, then it does not display a
+header line.
+
+@node Emulating Mode Line
+@subsection Emulating Mode-Line Formatting
+
+  You can use the function @code{format-mode-line} to compute
+the text that would appear in a mode line or header line
+based on a certain mode-line specification.
+
+@defun format-mode-line format &optional face window buffer
+This function formats a line of text according to @var{format} as if
+it were generating the mode line for @var{window}, but instead of
+displaying the text in the mode line or the header line, it returns
+the text as a string.  The argument @var{window} defaults to the
+selected window.  If @var{buffer} is non-@code{nil}, all the
+information used is taken from @var{buffer}; by default, it comes from
+@var{window}'s buffer.
+
+The value string normally has text properties that correspond to the
+faces, keymaps, etc., that the mode line would have.  And any character
+for which no @code{face} property is specified gets a default
+value which is usually @var{face}.  (If @var{face} is @code{t},
+that stands for either @code{mode-line} if @var{window} is selected,
+otherwise @code{mode-line-inactive}.  If @var{face} is @code{nil} or
+omitted, that stands for no face property.)
+
+However, if @var{face} is an integer, the value has no text properties.
+
+For example, @code{(format-mode-line header-line-format)} returns the
+text that would appear in the selected window's header line (@code{""}
+if it has no header line).  @code{(format-mode-line header-line-format
+'header-line)} returns the same text, with each character
+carrying the face that it will have in the header line itself.
+@end defun
+
+@node Imenu
+@section Imenu
+
+@cindex Imenu
+  @dfn{Imenu} is a feature that lets users select a definition or
+section in the buffer, from a menu which lists all of them, to go
+directly to that location in the buffer.  Imenu works by constructing
+a buffer index which lists the names and buffer positions of the
+definitions, or other named portions of the buffer; then the user can
+choose one of them and move point to it.  Major modes can add a menu
+bar item to use Imenu using @code{imenu-add-to-menubar}.
+
+@defun imenu-add-to-menubar name
+This function defines a local menu bar item named @var{name}
+to run Imenu.
+@end defun
+
+  The user-level commands for using Imenu are described in the Emacs
+Manual (@pxref{Imenu,, Imenu, emacs, the Emacs Manual}).  This section
+explains how to customize Imenu's method of finding definitions or
+buffer portions for a particular major mode.
+
+  The usual and simplest way is to set the variable
+@code{imenu-generic-expression}:
+
+@defvar imenu-generic-expression
+This variable, if non-@code{nil}, is a list that specifies regular
+expressions for finding definitions for Imenu.  Simple elements of
+@code{imenu-generic-expression} look like this:
+
+@example
+(@var{menu-title} @var{regexp} @var{index})
+@end example
+
+Here, if @var{menu-title} is non-@code{nil}, it says that the matches
+for this element should go in a submenu of the buffer index;
+@var{menu-title} itself specifies the name for the submenu.  If
+@var{menu-title} is @code{nil}, the matches for this element go directly
+in the top level of the buffer index.
+
+The second item in the list, @var{regexp}, is a regular expression
+(@pxref{Regular Expressions}); anything in the buffer that it matches
+is considered a definition, something to mention in the buffer index.
+The third item, @var{index}, is a non-negative integer that indicates
+which subexpression in @var{regexp} matches the definition's name.
+
+An element can also look like this:
+
+@example
+(@var{menu-title} @var{regexp} @var{index} @var{function} @var{arguments}@dots{})
+@end example
+
+Each match for this element creates an index item, and when the index
+item is selected by the user, it calls @var{function} with arguments
+consisting of the item name, the buffer position, and @var{arguments}.
+
+For Emacs Lisp mode, @code{imenu-generic-expression} could look like
+this:
+
+@c should probably use imenu-syntax-alist and \\sw rather than [-A-Za-z0-9+]
+@example
+@group
+((nil "^\\s-*(def\\(un\\|subst\\|macro\\|advice\\)\
+\\s-+\\([-A-Za-z0-9+]+\\)" 2)
+@end group
+@group
+ ("*Vars*" "^\\s-*(def\\(var\\|const\\)\
+\\s-+\\([-A-Za-z0-9+]+\\)" 2)
+@end group
+@group
+ ("*Types*"
+  "^\\s-*\
+(def\\(type\\|struct\\|class\\|ine-condition\\)\
+\\s-+\\([-A-Za-z0-9+]+\\)" 2))
+@end group
+@end example
+
+Setting this variable makes it buffer-local in the current buffer.
+@end defvar
+
+@defvar imenu-case-fold-search
+This variable controls whether matching against the regular
+expressions in the value of @code{imenu-generic-expression} is
+case-sensitive: @code{t}, the default, means matching should ignore
+case.
+
+Setting this variable makes it buffer-local in the current buffer.
+@end defvar
+
+@defvar imenu-syntax-alist
+This variable is an alist of syntax table modifiers to use while
+processing @code{imenu-generic-expression}, to override the syntax table
+of the current buffer.  Each element should have this form:
+
+@example
+(@var{characters} . @var{syntax-description})
+@end example
+
+The @sc{car}, @var{characters}, can be either a character or a string.
+The element says to give that character or characters the syntax
+specified by @var{syntax-description}, which is passed to
+@code{modify-syntax-entry} (@pxref{Syntax Table Functions}).
+
+This feature is typically used to give word syntax to characters which
+normally have symbol syntax, and thus to simplify
+@code{imenu-generic-expression} and speed up matching.
+For example, Fortran mode uses it this way:
+
+@example
+(setq imenu-syntax-alist '(("_$" . "w")))
+@end example
+
+The @code{imenu-generic-expression} regular expressions can then use
+@samp{\\sw+} instead of @samp{\\(\\sw\\|\\s_\\)+}.  Note that this
+technique may be inconvenient when the mode needs to limit the initial
+character of a name to a smaller set of characters than are allowed in
+the rest of a name.
+
+Setting this variable makes it buffer-local in the current buffer.
+@end defvar
+
+  Another way to customize Imenu for a major mode is to set the
+variables @code{imenu-prev-index-position-function} and
+@code{imenu-extract-index-name-function}:
+
+@defvar imenu-prev-index-position-function
+If this variable is non-@code{nil}, its value should be a function that
+finds the next ``definition'' to put in the buffer index, scanning
+backward in the buffer from point.  It should return @code{nil} if it
+doesn't find another ``definition'' before point.  Otherwise it should
+leave point at the place it finds a ``definition'' and return any
+non-@code{nil} value.
+
+Setting this variable makes it buffer-local in the current buffer.
+@end defvar
+
+@defvar imenu-extract-index-name-function
+If this variable is non-@code{nil}, its value should be a function to
+return the name for a definition, assuming point is in that definition
+as the @code{imenu-prev-index-position-function} function would leave
+it.
+
+Setting this variable makes it buffer-local in the current buffer.
+@end defvar
+
+  The last way to customize Imenu for a major mode is to set the
+variable @code{imenu-create-index-function}:
+
+@defvar imenu-create-index-function
+This variable specifies the function to use for creating a buffer
+index.  The function should take no arguments, and return an index
+alist for the current buffer.  It is called within
+@code{save-excursion}, so where it leaves point makes no difference.
+
+The index alist can have three types of elements.  Simple elements
+look like this:
+
+@example
+(@var{index-name} . @var{index-position})
+@end example
+
+Selecting a simple element has the effect of moving to position
+@var{index-position} in the buffer.  Special elements look like this:
+
+@example
+(@var{index-name} @var{index-position} @var{function} @var{arguments}@dots{})
+@end example
+
+Selecting a special element performs:
+
+@example
+(funcall @var{function}
+         @var{index-name} @var{index-position} @var{arguments}@dots{})
+@end example
+
+A nested sub-alist element looks like this:
+
+@example
+(@var{menu-title} @var{sub-alist})
+@end example
+
+It creates the submenu @var{menu-title} specified by @var{sub-alist}.
+
+The default value of @code{imenu-create-index-function} is
+@code{imenu-default-create-index-function}.  This function calls the
+value of @code{imenu-prev-index-position-function} and the value of
+@code{imenu-extract-index-name-function} to produce the index alist.
+However, if either of these two variables is @code{nil}, the default
+function uses @code{imenu-generic-expression} instead.
+
+Setting this variable makes it buffer-local in the current buffer.
+@end defvar
+
+@node Font Lock Mode
+@section Font Lock Mode
+@cindex Font Lock mode
+
+  @dfn{Font Lock mode} is a feature that automatically attaches
+@code{face} properties to certain parts of the buffer based on their
+syntactic role.  How it parses the buffer depends on the major mode;
+most major modes define syntactic criteria for which faces to use in
+which contexts.  This section explains how to customize Font Lock for a
+particular major mode.
+
+  Font Lock mode finds text to highlight in two ways: through
+syntactic parsing based on the syntax table, and through searching
+(usually for regular expressions).  Syntactic fontification happens
+first; it finds comments and string constants and highlights them.
+Search-based fontification happens second.
+
+@menu
+* Font Lock Basics::            Overview of customizing Font Lock.
+* Search-based Fontification::  Fontification based on regexps.
+* Customizing Keywords::        Customizing search-based fontification.
+* Other Font Lock Variables::   Additional customization facilities.
+* Levels of Font Lock::         Each mode can define alternative levels
+                                  so that the user can select more or less.
+* Precalculated Fontification:: How Lisp programs that produce the buffer
+                                  contents can also specify how to fontify it.
+* Faces for Font Lock::         Special faces specifically for Font Lock.
+* Syntactic Font Lock::         Fontification based on syntax tables.
+* Setting Syntax Properties::   Defining character syntax based on context
+                                  using the Font Lock mechanism.
+* Multiline Font Lock::         How to coerce Font Lock into properly
+                                  highlighting multiline constructs.
+@end menu
+
+@node Font Lock Basics
+@subsection Font Lock Basics
+
+  There are several variables that control how Font Lock mode highlights
+text.  But major modes should not set any of these variables directly.
+Instead, they should set @code{font-lock-defaults} as a buffer-local
+variable.  The value assigned to this variable is used, if and when Font
+Lock mode is enabled, to set all the other variables.
+
+@defvar font-lock-defaults
+This variable is set by major modes, as a buffer-local variable, to
+specify how to fontify text in that mode.  It automatically becomes
+buffer-local when you set it.  If its value is @code{nil}, Font-Lock
+mode does no highlighting, and you can use the @samp{Faces} menu
+(under @samp{Edit} and then @samp{Text Properties} in the menu bar) to
+assign faces explicitly to text in the buffer.
+
+If non-@code{nil}, the value should look like this:
+
+@example
+(@var{keywords} [@var{keywords-only} [@var{case-fold}
+ [@var{syntax-alist} [@var{syntax-begin} @var{other-vars}@dots{}]]]])
+@end example
+
+The first element, @var{keywords}, indirectly specifies the value of
+@code{font-lock-keywords} which directs search-based fontification.
+It can be a symbol, a variable or a function whose value is the list
+to use for @code{font-lock-keywords}.  It can also be a list of
+several such symbols, one for each possible level of fontification.
+The first symbol specifies how to do level 1 fontification, the second
+symbol how to do level 2, and so on.  @xref{Levels of Font Lock}.
+
+The second element, @var{keywords-only}, specifies the value of the
+variable @code{font-lock-keywords-only}.  If this is omitted or
+@code{nil}, syntactic fontification (of strings and comments) is also
+performed.  If this is non-@code{nil}, such fontification is not
+performed.  @xref{Syntactic Font Lock}.
+
+The third element, @var{case-fold}, specifies the value of
+@code{font-lock-keywords-case-fold-search}.  If it is non-@code{nil},
+Font Lock mode ignores case when searching as directed by
+@code{font-lock-keywords}.
+
+If the fourth element, @var{syntax-alist}, is non-@code{nil}, it
+should be a list of cons cells of the form @code{(@var{char-or-string}
+. @var{string})}.  These are used to set up a syntax table for
+syntactic fontification (@pxref{Syntax Table Functions}).  The
+resulting syntax table is stored in @code{font-lock-syntax-table}.
+
+The fifth element, @var{syntax-begin}, specifies the value of
+@code{font-lock-beginning-of-syntax-function}.  We recommend setting
+this variable to @code{nil} and using @code{syntax-begin-function}
+instead.
+
+All the remaining elements (if any) are collectively called
+@var{other-vars}.  Each of these elements should have the form
+@code{(@var{variable} . @var{value})}---which means, make
+@var{variable} buffer-local and then set it to @var{value}.  You can
+use these @var{other-vars} to set other variables that affect
+fontification, aside from those you can control with the first five
+elements.  @xref{Other Font Lock Variables}.
+@end defvar
+
+  If your mode fontifies text explicitly by adding
+@code{font-lock-face} properties, it can specify @code{(nil t)} for
+@code{font-lock-defaults} to turn off all automatic fontification.
+However, this is not required; it is possible to fontify some things
+using @code{font-lock-face} properties and set up automatic
+fontification for other parts of the text.
+
+@node Search-based Fontification
+@subsection Search-based Fontification
+
+  The most important variable for customizing Font Lock mode is
+@code{font-lock-keywords}.  It specifies the search criteria for
+search-based fontification.  You should specify the value of this
+variable with @var{keywords} in @code{font-lock-defaults}.
+
+@defvar font-lock-keywords
+This variable's value is a list of the keywords to highlight.  Be
+careful when composing regular expressions for this list; a poorly
+written pattern can dramatically slow things down!
+@end defvar
+
+  Each element of @code{font-lock-keywords} specifies how to find
+certain cases of text, and how to highlight those cases.  Font Lock mode
+processes the elements of @code{font-lock-keywords} one by one, and for
+each element, it finds and handles all matches.  Ordinarily, once
+part of the text has been fontified already, this cannot be overridden
+by a subsequent match in the same text; but you can specify different
+behavior using the @var{override} element of a @var{subexp-highlighter}.
+
+  Each element of @code{font-lock-keywords} should have one of these
+forms:
+
+@table @code
+@item @var{regexp}
+Highlight all matches for @var{regexp} using
+@code{font-lock-keyword-face}.  For example,
+
+@example
+;; @r{Highlight occurrences of the word @samp{foo}}
+;; @r{using @code{font-lock-keyword-face}.}
+"\\<foo\\>"
+@end example
+
+The function @code{regexp-opt} (@pxref{Regexp Functions}) is useful
+for calculating optimal regular expressions to match a number of
+different keywords.
+
+@item @var{function}
+Find text by calling @var{function}, and highlight the matches
+it finds using @code{font-lock-keyword-face}.
+
+When @var{function} is called, it receives one argument, the limit of
+the search; it should begin searching at point, and not search beyond the
+limit.  It should return non-@code{nil} if it succeeds, and set the
+match data to describe the match that was found.  Returning @code{nil}
+indicates failure of the search.
+
+Fontification will call @var{function} repeatedly with the same limit,
+and with point where the previous invocation left it, until
+@var{function} fails.  On failure, @var{function} need not reset point
+in any particular way.
+
+@item (@var{matcher} . @var{subexp})
+In this kind of element, @var{matcher} is either a regular
+expression or a function, as described above.  The @sc{cdr},
+@var{subexp}, specifies which subexpression of @var{matcher} should be
+highlighted (instead of the entire text that @var{matcher} matched).
+
+@example
+;; @r{Highlight the @samp{bar} in each occurrence of @samp{fubar},}
+;; @r{using @code{font-lock-keyword-face}.}
+("fu\\(bar\\)" . 1)
+@end example
+
+If you use @code{regexp-opt} to produce the regular expression
+@var{matcher}, you can use @code{regexp-opt-depth} (@pxref{Regexp
+Functions}) to calculate the value for @var{subexp}.
+
+@item (@var{matcher} . @var{facespec})
+In this kind of element, @var{facespec} is an expression whose value
+specifies the face to use for highlighting.  In the simplest case,
+@var{facespec} is a Lisp variable (a symbol) whose value is a face
+name.
+
+@example
+;; @r{Highlight occurrences of @samp{fubar},}
+;; @r{using the face which is the value of @code{fubar-face}.}
+("fubar" . fubar-face)
+@end example
+
+However, @var{facespec} can also evaluate to a list of this form:
+
+@example
+(face @var{face} @var{prop1} @var{val1} @var{prop2} @var{val2}@dots{})
+@end example
+
+@noindent
+to specify the face @var{face} and various additional text properties
+to put on the text that matches.  If you do this, be sure to add the
+other text property names that you set in this way to the value of
+@code{font-lock-extra-managed-props} so that the properties will also
+be cleared out when they are no longer appropriate.  Alternatively,
+you can set the variable @code{font-lock-unfontify-region-function} to
+a function that clears these properties.  @xref{Other Font Lock
+Variables}.
+
+@item (@var{matcher} . @var{subexp-highlighter})
+In this kind of element, @var{subexp-highlighter} is a list
+which specifies how to highlight matches found by @var{matcher}.
+It has the form:
+
+@example
+(@var{subexp} @var{facespec} [[@var{override} [@var{laxmatch}]])
+@end example
+
+The @sc{car}, @var{subexp}, is an integer specifying which subexpression
+of the match to fontify (0 means the entire matching text).  The second
+subelement, @var{facespec}, is an expression whose value specifies the
+face, as described above.
+
+The last two values in @var{subexp-highlighter}, @var{override} and
+@var{laxmatch}, are optional flags.  If @var{override} is @code{t},
+this element can override existing fontification made by previous
+elements of @code{font-lock-keywords}.  If it is @code{keep}, then
+each character is fontified if it has not been fontified already by
+some other element.  If it is @code{prepend}, the face specified by
+@var{facespec} is added to the beginning of the @code{font-lock-face}
+property.  If it is @code{append}, the face is added to the end of the
+@code{font-lock-face} property.
+
+If @var{laxmatch} is non-@code{nil}, it means there should be no error
+if there is no subexpression numbered @var{subexp} in @var{matcher}.
+Obviously, fontification of the subexpression numbered @var{subexp} will
+not occur.  However, fontification of other subexpressions (and other
+regexps) will continue.  If @var{laxmatch} is @code{nil}, and the
+specified subexpression is missing, then an error is signaled which
+terminates search-based fontification.
+
+Here are some examples of elements of this kind, and what they do:
+
+@smallexample
+;; @r{Highlight occurrences of either @samp{foo} or @samp{bar}, using}
+;; @r{@code{foo-bar-face}, even if they have already been highlighted.}
+;; @r{@code{foo-bar-face} should be a variable whose value is a face.}
+("foo\\|bar" 0 foo-bar-face t)
+
+;; @r{Highlight the first subexpression within each occurrence}
+;; @r{that the function @code{fubar-match} finds,}
+;; @r{using the face which is the value of @code{fubar-face}.}
+(fubar-match 1 fubar-face)
+@end smallexample
+
+@item (@var{matcher} . @var{anchored-highlighter})
+In this kind of element, @var{anchored-highlighter} specifies how to
+highlight text that follows a match found by @var{matcher}.  So a
+match found by @var{matcher} acts as the anchor for further searches
+specified by @var{anchored-highlighter}.  @var{anchored-highlighter}
+is a list of the following form:
+
+@example
+(@var{anchored-matcher} @var{pre-form} @var{post-form}
+                        @var{subexp-highlighters}@dots{})
+@end example
+
+Here, @var{anchored-matcher}, like @var{matcher}, is either a regular
+expression or a function.  After a match of @var{matcher} is found,
+point is at the end of the match.  Now, Font Lock evaluates the form
+@var{pre-form}.  Then it searches for matches of
+@var{anchored-matcher} and uses @var{subexp-highlighters} to highlight
+these.  A @var{subexp-highlighter} is as described above.  Finally,
+Font Lock evaluates @var{post-form}.
+
+The forms @var{pre-form} and @var{post-form} can be used to initialize
+before, and cleanup after, @var{anchored-matcher} is used.  Typically,
+@var{pre-form} is used to move point to some position relative to the
+match of @var{matcher}, before starting with @var{anchored-matcher}.
+@var{post-form} might be used to move back, before resuming with
+@var{matcher}.
+
+After Font Lock evaluates @var{pre-form}, it does not search for
+@var{anchored-matcher} beyond the end of the line.  However, if
+@var{pre-form} returns a buffer position that is greater than the
+position of point after @var{pre-form} is evaluated, then the position
+returned by @var{pre-form} is used as the limit of the search instead.
+It is generally a bad idea to return a position greater than the end
+of the line; in other words, the @var{anchored-matcher} search should
+not span lines.
+
+For example,
+
+@smallexample
+;; @r{Highlight occurrences of the word @samp{item} following}
+;; @r{an occurrence of the word @samp{anchor} (on the same line)}
+;; @r{in the value of @code{item-face}.}
+("\\<anchor\\>" "\\<item\\>" nil nil (0 item-face))
+@end smallexample
+
+Here, @var{pre-form} and @var{post-form} are @code{nil}.  Therefore
+searching for @samp{item} starts at the end of the match of
+@samp{anchor}, and searching for subsequent instances of @samp{anchor}
+resumes from where searching for @samp{item} concluded.
+
+@item (@var{matcher} @var{highlighters}@dots{})
+This sort of element specifies several @var{highlighter} lists for a
+single @var{matcher}.  A @var{highlighter} list can be of the type
+@var{subexp-highlighter} or @var{anchored-highlighter} as described
+above.
+
+For example,
+
+@smallexample
+;; @r{Highlight occurrences of the word @samp{anchor} in the value}
+;; @r{of @code{anchor-face}, and subsequent occurrences of the word}
+;; @r{@samp{item} (on the same line) in the value of @code{item-face}.}
+("\\<anchor\\>" (0 anchor-face)
+                ("\\<item\\>" nil nil (0 item-face)))
+@end smallexample
+
+@item (eval . @var{form})
+Here @var{form} is an expression to be evaluated the first time
+this value of @code{font-lock-keywords} is used in a buffer.
+Its value should have one of the forms described in this table.
+@end table
+
+@strong{Warning:} Do not design an element of @code{font-lock-keywords}
+to match text which spans lines; this does not work reliably.
+For details, see @xref{Multiline Font Lock}.
+
+You can use @var{case-fold} in @code{font-lock-defaults} to specify
+the value of @code{font-lock-keywords-case-fold-search} which says
+whether search-based fontification should be case-insensitive.
+
+@defvar font-lock-keywords-case-fold-search
+Non-@code{nil} means that regular expression matching for the sake of
+@code{font-lock-keywords} should be case-insensitive.
+@end defvar
+
+@node Customizing Keywords
+@subsection Customizing Search-Based Fontification
+
+  You can use @code{font-lock-add-keywords} to add additional
+search-based fontification rules to a major mode, and
+@code{font-lock-remove-keywords} to removes rules.
+
+@defun font-lock-add-keywords mode keywords &optional how
+This function adds highlighting @var{keywords}, for the current buffer
+or for major mode @var{mode}.  The argument @var{keywords} should be a
+list with the same format as the variable @code{font-lock-keywords}.
+
+If @var{mode} is a symbol which is a major mode command name, such as
+@code{c-mode}, the effect is that enabling Font Lock mode in
+@var{mode} will add @var{keywords} to @code{font-lock-keywords}.
+Calling with a non-@code{nil} value of @var{mode} is correct only in
+your @file{~/.emacs} file.
+
+If @var{mode} is @code{nil}, this function adds @var{keywords} to
+@code{font-lock-keywords} in the current buffer.  This way of calling
+@code{font-lock-add-keywords} is usually used in mode hook functions.
+
+By default, @var{keywords} are added at the beginning of
+@code{font-lock-keywords}.  If the optional argument @var{how} is
+@code{set}, they are used to replace the value of
+@code{font-lock-keywords}.  If @var{how} is any other non-@code{nil}
+value, they are added at the end of @code{font-lock-keywords}.
+
+Some modes provide specialized support you can use in additional
+highlighting patterns.  See the variables
+@code{c-font-lock-extra-types}, @code{c++-font-lock-extra-types},
+and @code{java-font-lock-extra-types}, for example.
+
+@strong{Warning:} major mode functions must not call
+@code{font-lock-add-keywords} under any circumstances, either directly
+or indirectly, except through their mode hooks.  (Doing so would lead
+to incorrect behavior for some minor modes.)  They should set up their
+rules for search-based fontification by setting
+@code{font-lock-keywords}.
+@end defun
+
+@defun font-lock-remove-keywords mode keywords
+This function removes @var{keywords} from @code{font-lock-keywords}
+for the current buffer or for major mode @var{mode}.  As in
+@code{font-lock-add-keywords}, @var{mode} should be a major mode
+command name or @code{nil}.  All the caveats and requirements for
+@code{font-lock-add-keywords} apply here too.
+@end defun
+
+  For example, this code
+
+@smallexample
+(font-lock-add-keywords 'c-mode
+ '(("\\<\\(FIXME\\):" 1 font-lock-warning-face prepend)
+   ("\\<\\(and\\|or\\|not\\)\\>" . font-lock-keyword-face)))
+@end smallexample
+
+@noindent
+adds two fontification patterns for C mode: one to fontify the word
+@samp{FIXME}, even in comments, and another to fontify the words
+@samp{and}, @samp{or} and @samp{not} as keywords.
+
+@noindent
+That example affects only C mode proper.  To add the same patterns to
+C mode @emph{and} all modes derived from it, do this instead:
+
+@smallexample
+(add-hook 'c-mode-hook
+ (lambda ()
+  (font-lock-add-keywords nil
+   '(("\\<\\(FIXME\\):" 1 font-lock-warning-face prepend)
+     ("\\<\\(and\\|or\\|not\\)\\>" .
+      font-lock-keyword-face)))))
+@end smallexample
+
+@node Other Font Lock Variables
+@subsection Other Font Lock Variables
+
+  This section describes additional variables that a major mode can
+set by means of @var{other-vars} in @code{font-lock-defaults}
+(@pxref{Font Lock Basics}).
+
+@defvar font-lock-mark-block-function
+If this variable is non-@code{nil}, it should be a function that is
+called with no arguments, to choose an enclosing range of text for
+refontification for the command @kbd{M-o M-o}
+(@code{font-lock-fontify-block}).
+
+The function should report its choice by placing the region around it.
+A good choice is a range of text large enough to give proper results,
+but not too large so that refontification becomes slow.  Typical values
+are @code{mark-defun} for programming modes or @code{mark-paragraph} for
+textual modes.
+@end defvar
+
+@defvar font-lock-extra-managed-props
+This variable specifies additional properties (other than
+@code{font-lock-face}) that are being managed by Font Lock mode.  It
+is used by @code{font-lock-default-unfontify-region}, which normally
+only manages the @code{font-lock-face} property.  If you want Font
+Lock to manage other properties as well, you must specify them in a
+@var{facespec} in @code{font-lock-keywords} as well as add them to
+this list.  @xref{Search-based Fontification}.
+@end defvar
+
+@defvar font-lock-fontify-buffer-function
+Function to use for fontifying the buffer.  The default value is
+@code{font-lock-default-fontify-buffer}.
+@end defvar
+
+@defvar font-lock-unfontify-buffer-function
+Function to use for unfontifying the buffer.  This is used when
+turning off Font Lock mode.  The default value is
+@code{font-lock-default-unfontify-buffer}.
+@end defvar
+
+@defvar font-lock-fontify-region-function
+Function to use for fontifying a region.  It should take two
+arguments, the beginning and end of the region, and an optional third
+argument @var{verbose}.  If @var{verbose} is non-@code{nil}, the
+function should print status messages.  The default value is
+@code{font-lock-default-fontify-region}.
+@end defvar
+
+@defvar font-lock-unfontify-region-function
+Function to use for unfontifying a region.  It should take two
+arguments, the beginning and end of the region.  The default value is
+@code{font-lock-default-unfontify-region}.
+@end defvar
+
+@ignore
+@defvar font-lock-inhibit-thing-lock
+List of Font Lock mode related modes that should not be turned on.
+Currently, valid mode names are @code{fast-lock-mode},
+@code{jit-lock-mode} and @code{lazy-lock-mode}.
+@end defvar
+@end ignore
+
+@node Levels of Font Lock
+@subsection Levels of Font Lock
+
+  Many major modes offer three different levels of fontification.  You
+can define multiple levels by using a list of symbols for @var{keywords}
+in @code{font-lock-defaults}.  Each symbol specifies one level of
+fontification; it is up to the user to choose one of these levels.  The
+chosen level's symbol value is used to initialize
+@code{font-lock-keywords}.
+
+  Here are the conventions for how to define the levels of
+fontification:
+
+@itemize @bullet
+@item
+Level 1: highlight function declarations, file directives (such as include or
+import directives), strings and comments.  The idea is speed, so only
+the most important and top-level components are fontified.
+
+@item
+Level 2: in addition to level 1, highlight all language keywords,
+including type names that act like keywords, as well as named constant
+values.  The idea is that all keywords (either syntactic or semantic)
+should be fontified appropriately.
+
+@item
+Level 3: in addition to level 2, highlight the symbols being defined in
+function and variable declarations, and all builtin function names,
+wherever they appear.
+@end itemize
+
+@node Precalculated Fontification
+@subsection Precalculated Fontification
+
+  In addition to using @code{font-lock-defaults} for search-based
+fontification, you may use the special character property
+@code{font-lock-face} (@pxref{Special Properties}).  This property
+acts just like the explicit @code{face} property, but its activation
+is toggled when the user calls @kbd{M-x font-lock-mode}.  Using
+@code{font-lock-face} is especially convenient for special modes
+which construct their text programmatically, such as
+@code{list-buffers} and @code{occur}.
+
+If your mode does not use any of the other machinery of Font Lock
+(i.e. it only uses the @code{font-lock-face} property), it should not
+set the variable @code{font-lock-defaults}.
+
+@node Faces for Font Lock
+@subsection Faces for Font Lock
+@cindex faces for font lock
+@cindex font lock faces
+
+  You can make Font Lock mode use any face, but several faces are
+defined specifically for Font Lock mode.  Each of these symbols is both
+a face name, and a variable whose default value is the symbol itself.
+Thus, the default value of @code{font-lock-comment-face} is
+@code{font-lock-comment-face}.  This means you can write
+@code{font-lock-comment-face} in a context such as
+@code{font-lock-keywords} where a face-name-valued expression is used.
+
+@table @code
+@item font-lock-comment-face
+@vindex font-lock-comment-face
+Used (typically) for comments.
+
+@item font-lock-comment-delimiter-face
+@vindex font-lock-comment-delimiter-face
+Used (typically) for comments delimiters.
+
+@item font-lock-doc-face
+@vindex font-lock-doc-face
+Used (typically) for documentation strings in the code.
+
+@item font-lock-string-face
+@vindex font-lock-string-face
+Used (typically) for string constants.
+
+@item font-lock-keyword-face
+@vindex font-lock-keyword-face
+Used (typically) for keywords---names that have special syntactic
+significance, like @code{for} and @code{if} in C.
+
+@item font-lock-builtin-face
+@vindex font-lock-builtin-face
+Used (typically) for built-in function names.
+
+@item font-lock-function-name-face
+@vindex font-lock-function-name-face
+Used (typically) for the name of a function being defined or declared,
+in a function definition or declaration.
+
+@item font-lock-variable-name-face
+@vindex font-lock-variable-name-face
+Used (typically) for the name of a variable being defined or declared,
+in a variable definition or declaration.
+
+@item font-lock-type-face
+@vindex font-lock-type-face
+Used (typically) for names of user-defined data types,
+where they are defined and where they are used.
+
+@item font-lock-constant-face
+@vindex font-lock-constant-face
+Used (typically) for constant names.
+
+@item font-lock-preprocessor-face
+@vindex font-lock-preprocessor-face
+Used (typically) for preprocessor commands.
+
+@item font-lock-negation-char-face
+@vindex font-lock-negation-char-face
+Used (typically) for easily-overlooked negation characters.
+
+@item font-lock-warning-face
+@vindex font-lock-warning-face
+Used (typically) for constructs that are peculiar, or that greatly
+change the meaning of other text.  For example, this is used for
+@samp{;;;###autoload} cookies in Emacs Lisp, and for @code{#error}
+directives in C.
+@end table
+
+@node Syntactic Font Lock
+@subsection Syntactic Font Lock
+@cindex syntactic font lock
+
+Syntactic fontification uses the syntax table to find comments and
+string constants (@pxref{Syntax Tables}).  It highlights them using
+@code{font-lock-comment-face} and @code{font-lock-string-face}
+(@pxref{Faces for Font Lock}), or whatever
+@code{font-lock-syntactic-face-function} chooses.  There are several
+variables that affect syntactic fontification; you should set them by
+means of @code{font-lock-defaults} (@pxref{Font Lock Basics}).
+
+@defvar font-lock-keywords-only
+Non-@code{nil} means Font Lock should not do syntactic fontification;
+it should only fontify based on @code{font-lock-keywords}.  The normal
+way for a mode to set this variable to @code{t} is with
+@var{keywords-only} in @code{font-lock-defaults}.
+@end defvar
+
+@defvar font-lock-syntax-table
+This variable holds the syntax table to use for fontification of
+comments and strings.  Specify it using @var{syntax-alist} in
+@code{font-lock-defaults}.  If this is @code{nil}, fontification uses
+the buffer's syntax table.
+@end defvar
+
+@defvar font-lock-beginning-of-syntax-function
+If this variable is non-@code{nil}, it should be a function to move
+point back to a position that is syntactically at ``top level'' and
+outside of strings or comments.  Font Lock uses this when necessary
+to get the right results for syntactic fontification.
+
+This function is called with no arguments.  It should leave point at
+the beginning of any enclosing syntactic block.  Typical values are
+@code{beginning-of-line} (used when the start of the line is known to
+be outside a syntactic block), or @code{beginning-of-defun} for
+programming modes, or @code{backward-paragraph} for textual modes.
+
+If the value is @code{nil}, Font Lock uses
+@code{syntax-begin-function} to move back outside of any comment,
+string, or sexp.  This variable is semi-obsolete; we recommend setting
+@code{syntax-begin-function} instead.
+
+Specify this variable using @var{syntax-begin} in
+@code{font-lock-defaults}.
+@end defvar
+
+@defvar font-lock-syntactic-face-function
+A function to determine which face to use for a given syntactic
+element (a string or a comment).  The function is called with one
+argument, the parse state at point returned by
+@code{parse-partial-sexp}, and should return a face.  The default
+value returns @code{font-lock-comment-face} for comments and
+@code{font-lock-string-face} for strings.
+
+This can be used to highlighting different kinds of strings or
+comments differently.  It is also sometimes abused together with
+@code{font-lock-syntactic-keywords} to highlight constructs that span
+multiple lines, but this is too esoteric to document here.
+
+Specify this variable using @var{other-vars} in
+@code{font-lock-defaults}.
+@end defvar
+
+@node Setting Syntax Properties
+@subsection Setting Syntax Properties
+
+  Font Lock mode can be used to update @code{syntax-table} properties
+automatically (@pxref{Syntax Properties}).  This is useful in
+languages for which a single syntax table by itself is not sufficient.
+
+@defvar font-lock-syntactic-keywords
+This variable enables and controls updating @code{syntax-table}
+properties by Font Lock.  Its value should be a list of elements of
+this form:
+
+@example
+(@var{matcher} @var{subexp} @var{syntax} @var{override} @var{laxmatch})
+@end example
+
+The parts of this element have the same meanings as in the corresponding
+sort of element of @code{font-lock-keywords},
+
+@example
+(@var{matcher} @var{subexp} @var{facespec} @var{override} @var{laxmatch})
+@end example
+
+However, instead of specifying the value @var{facespec} to use for the
+@code{face} property, it specifies the value @var{syntax} to use for
+the @code{syntax-table} property.  Here, @var{syntax} can be a string
+(as taken by @code{modify-syntax-entry}), a syntax table, a cons cell
+(as returned by @code{string-to-syntax}), or an expression whose value
+is one of those two types.  @var{override} cannot be @code{prepend} or
+@code{append}.
+
+For example, an element of the form:
+
+@example
+("\\$\\(#\\)" 1 ".")
+@end example
+
+highlights syntactically a hash character when following a dollar
+character, with a SYNTAX of @code{"."} (meaning punctuation syntax).
+Assuming that the buffer syntax table specifies hash characters to
+have comment start syntax, the element will only highlight hash
+characters that do not follow dollar characters as comments
+syntactically.
+
+An element of the form:
+
+@example
+ ("\\('\\).\\('\\)"
+  (1 "\"")
+  (2 "\""))
+@end example
+
+highlights syntactically both single quotes which surround a single
+character, with a SYNTAX of @code{"\""} (meaning string quote syntax).
+Assuming that the buffer syntax table does not specify single quotes
+to have quote syntax, the element will only highlight single quotes of
+the form @samp{'@var{c}'} as strings syntactically.  Other forms, such
+as @samp{foo'bar} or @samp{'fubar'}, will not be highlighted as
+strings.
+
+Major modes normally set this variable with @var{other-vars} in
+@code{font-lock-defaults}.
+@end defvar
+
+@node Multiline Font Lock
+@subsection Multiline Font Lock Constructs
+@cindex multiline font lock
+
+  Normally, elements of @code{font-lock-keywords} should not match
+across multiple lines; that doesn't work reliably, because Font Lock
+usually scans just part of the buffer, and it can miss a multi-line
+construct that crosses the line boundary where the scan starts.  (The
+scan normally starts at the beginning of a line.)
+
+  Making elements that match multiline constructs work properly has
+two aspects: correct @emph{identification} and correct
+@emph{rehighlighting}.  The first means that Font Lock finds all
+multiline constructs.  The second means that Font Lock will correctly
+rehighlight all the relevant text when a multiline construct is
+changed---for example, if some of the text that was previously part of
+a multiline construct ceases to be part of it.  The two aspects are
+closely related, and often getting one of them to work will appear to
+make the other also work.  However, for reliable results you must
+attend explicitly to both aspects.
+
+  There are three ways to ensure correct identification of multiline
+constructs:
+
+@itemize
+@item
+Add a function to @code{font-lock-extend-region-functions} that does
+the @emph{identification} and extends the scan so that the scanned
+text never starts or ends in the middle of a multiline construct.
+@item
+Use the @code{font-lock-fontify-region-function} hook similarly to
+extend the scan so that the scanned text never starts or ends in the
+middle of a multiline construct.
+@item
+Somehow identify the multiline construct right when it gets inserted
+into the buffer (or at any point after that but before font-lock
+tries to highlight it), and mark it with a @code{font-lock-multiline}
+which will instruct font-lock not to start or end the scan in the
+middle of the construct.
+@end itemize
+
+  There are three ways to do rehighlighting of multiline constructs:
+
+@itemize
+@item
+Place a @code{font-lock-multiline} property on the construct.  This
+will rehighlight the whole construct if any part of it is changed.  In
+some cases you can do this automatically by setting the
+@code{font-lock-multiline} variable, which see.
+@item
+Make sure @code{jit-lock-contextually} is set and rely on it doing its
+job.  This will only rehighlight the part of the construct that
+follows the actual change, and will do it after a short delay.
+This only works if the highlighting of the various parts of your
+multiline construct never depends on text in subsequent lines.
+Since @code{jit-lock-contextually} is activated by default, this can
+be an attractive solution.
+@item
+Place a @code{jit-lock-defer-multiline} property on the construct.
+This works only if @code{jit-lock-contextually} is used, and with the
+same delay before rehighlighting, but like @code{font-lock-multiline},
+it also handles the case where highlighting depends on
+subsequent lines.
+@end itemize
+
+@menu
+* Font Lock Multiline::         Marking multiline chunks with a text property
+* Region to Fontify::           Controlling which region gets refontified
+                                  after a buffer change.
+@end menu
+
+@node Font Lock Multiline
+@subsubsection Font Lock Multiline
+
+  One way to ensure reliable rehighlighting of multiline Font Lock
+constructs is to put on them the text property @code{font-lock-multiline}.
+It should be present and non-@code{nil} for text that is part of a
+multiline construct.
+
+  When Font Lock is about to highlight a range of text, it first
+extends the boundaries of the range as necessary so that they do not
+fall within text marked with the @code{font-lock-multiline} property.
+Then it removes any @code{font-lock-multiline} properties from the
+range, and highlights it.  The highlighting specification (mostly
+@code{font-lock-keywords}) must reinstall this property each time,
+whenever it is appropriate.
+
+  @strong{Warning:} don't use the @code{font-lock-multiline} property
+on large ranges of text, because that will make rehighlighting slow.
+
+@defvar font-lock-multiline
+If the @code{font-lock-multiline} variable is set to @code{t}, Font
+Lock will try to add the @code{font-lock-multiline} property
+automatically on multiline constructs.  This is not a universal
+solution, however, since it slows down Font Lock somewhat.  It can
+miss some multiline constructs, or make the property larger or smaller
+than necessary.
+
+For elements whose @var{matcher} is a function, the function should
+ensure that submatch 0 covers the whole relevant multiline construct,
+even if only a small subpart will be highlighted.  It is often just as
+easy to add the @code{font-lock-multiline} property by hand.
+@end defvar
+
+  The @code{font-lock-multiline} property is meant to ensure proper
+refontification; it does not automatically identify new multiline
+constructs.  Identifying the requires that Font-Lock operate on large
+enough chunks at a time.  This will happen by accident on many cases,
+which may give the impression that multiline constructs magically work.
+If you set the @code{font-lock-multiline} variable non-@code{nil},
+this impression will be even stronger, since the highlighting of those
+constructs which are found will be properly updated from then on.
+But that does not work reliably.
+
+  To find multiline constructs reliably, you must either manually
+place the @code{font-lock-multiline} property on the text before
+Font-Lock looks at it, or use
+@code{font-lock-fontify-region-function}.
+
+@node Region to Fontify
+@subsubsection Region to Fontify after a Buffer Change
+
+  When a buffer is changed, the region that Font Lock refontifies is
+by default the smallest sequence of whole lines that spans the change.
+While this works well most of the time, sometimes it doesn't---for
+example, when a change alters the syntactic meaning of text on an
+earlier line.
+
+  You can enlarge (or even reduce) the region to fontify by setting
+one the following variables:
+
+@defvar font-lock-extend-after-change-region-function
+This buffer-local variable is either @code{nil} or a function for
+Font-Lock to call to determine the region to scan and fontify.
+
+The function is given three parameters, the standard @var{beg},
+@var{end}, and @var{old-len} from after-change-functions
+(@pxref{Change Hooks}).  It should return either a cons of the
+beginning and end buffer positions (in that order) of the region to
+fontify, or @code{nil} (which means choose the region in the standard
+way).  This function needs to preserve point, the match-data, and the
+current restriction.  The region it returns may start or end in the
+middle of a line.
+
+Since this function is called after every buffer change, it should be
+reasonably fast.
+@end defvar
+
+@node Desktop Save Mode
+@section Desktop Save Mode
+@cindex desktop save mode
+
+@dfn{Desktop Save Mode} is a feature to save the state of Emacs from
+one session to another.  The user-level commands for using Desktop
+Save Mode are described in the GNU Emacs Manual (@pxref{Saving Emacs
+Sessions,,, emacs, the GNU Emacs Manual}).  Modes whose buffers visit
+a file, don't have to do anything to use this feature.
+
+For buffers not visiting a file to have their state saved, the major
+mode must bind the buffer local variable @code{desktop-save-buffer} to
+a non-@code{nil} value.
+
+@defvar desktop-save-buffer
+If this buffer-local variable is non-@code{nil}, the buffer will have
+its state saved in the desktop file at desktop save.  If the value is
+a function, it is called at desktop save with argument
+@var{desktop-dirname}, and its value is saved in the desktop file along
+with the state of the buffer for which it was called.  When file names
+are returned as part of the auxiliary information, they should be
+formatted using the call
+
+@example
+(desktop-file-name @var{file-name} @var{desktop-dirname})
+@end example
+
+@end defvar
+
+For buffers not visiting a file to be restored, the major mode must
+define a function to do the job, and that function must be listed in
+the alist @code{desktop-buffer-mode-handlers}.
+
+@defvar desktop-buffer-mode-handlers
+Alist with elements
+
+@example
+(@var{major-mode} . @var{restore-buffer-function})
+@end example
+
+The function @var{restore-buffer-function} will be called with
+argument list
+
+@example
+(@var{buffer-file-name} @var{buffer-name} @var{desktop-buffer-misc})
+@end example
+
+and it should return the restored buffer.
+Here @var{desktop-buffer-misc} is the value returned by the function
+optionally bound to @code{desktop-save-buffer}.
+@end defvar
+
+@ignore
+   arch-tag: 4c7bff41-36e6-4da6-9e7f-9b9289e27c8e
+@end ignore