3 files changed, 110 insertions, 7 deletions

diff --git a/doc/lispref/functions.texi b/doc/lispref/functions.texi
index 8835667b82d..7cc041fa77e 100644
--- a/doc/lispref/functions.texi
+++ b/doc/lispref/functions.texi
@@ -861,15 +861,18 @@ into a list.  @code{mapc} always returns @var{sequence}.
 
 @defun mapconcat function sequence separator
 @code{mapconcat} applies @var{function} to each element of
-@var{sequence}: the results, which must be strings, are concatenated.
-Between each pair of result strings, @code{mapconcat} inserts the string
-@var{separator}.  Usually @var{separator} contains a space or comma or
-other suitable punctuation.
+@var{sequence}; the results, which must be sequences of characters
+(strings, vectors, or lists), are concatenated into a single string
+return value.  Between each pair of result sequences, @code{mapconcat}
+inserts the characters from @var{separator}, which also must be a
+string, or a vector or list of characters.  @xref{Sequences Arrays
+Vectors}.
 
 The argument @var{function} must be a function that can take one
-argument and return a string.  The argument @var{sequence} can be any
-kind of sequence except a char-table; that is, a list, a vector, a
-bool-vector, or a string.
+argument and returns a sequence of characters: a string, a vector, or
+a list.  The argument @var{sequence} can be any kind of sequence
+except a char-table; that is, a list, a vector, a bool-vector, or a
+string.
 
 @example
 @group
diff --git a/doc/lispref/positions.texi b/doc/lispref/positions.texi
index 72b76ce5c8f..9daf5cef059 100644
--- a/doc/lispref/positions.texi
+++ b/doc/lispref/positions.texi
@@ -572,6 +572,18 @@ The value returned is the window line number point has moved to, with
 the top line in the window numbered 0.
 @end deffn
 
+@vindex move-to-window-group-line-function
+@defun move-to-window-group-line count
+This function is like @code{move-to-window-line}, except that when the
+selected window is a part of a group of windows (@pxref{Window
+Group}), @code{move-to-window-group-line} will move to a position with
+respect to the entire group, not just the single window.  This
+condition holds when the buffer local variable
+@code{move-to-window-group-line-function} is set to a function.  In
+this case, @code{move-to-window-group-line} calls the function with
+the argument @var{count}, then returns its result.
+@end defun
+
 @defun compute-motion from frompos to topos width offsets window
 This function scans the current buffer, calculating screen positions.
 It scans the buffer forward from position @var{from}, assuming that is
diff --git a/doc/lispref/windows.texi b/doc/lispref/windows.texi
index 5c7947eeca6..e45201b0570 100644
--- a/doc/lispref/windows.texi
+++ b/doc/lispref/windows.texi
@@ -133,6 +133,30 @@ This function returns the selected window (which is always a live
 window).
 @end defun
 
+@anchor{Window Group}Sometimes several windows collectively and
+cooperatively display a buffer, for example, under the management of
+Follow Mode (@pxref{Follow Mode,,, emacs}), where the windows together
+display a bigger portion of the buffer than one window could alone.
+It is often useful to consider such a @dfn{window group} as a single
+entity.  Several functions such as @code{window-group-start}
+(@pxref{Window Start and End}) allow you to do this by supplying, as
+an argument, one of the windows as a stand in for the whole group.
+
+@defun selected-window-group
+@vindex selected-window-group-function
+When the selected window is a member of a group of windows, this
+function returns a list of the windows in the group, ordered such that
+the first window in the list is displaying the earliest part of the
+buffer, and so on.  Otherwise the function returns a list containing
+just the selected window.
+
+The selected window is considered part of a group when the buffer
+local variable @code{selected-window-group-function} is set to a
+function.  In this case, @code{selected-window-group} calls it with no
+arguments and returns its result (which should be the list of windows
+in the group).
+@end defun
+
 @node Windows and Frames
 @section Windows and Frames
 
@@ -3098,6 +3122,17 @@ window-start position; if you move point, do not expect the window-start
 position to change in response until after the next redisplay.
 @end defun
 
+@defun window-group-start &optional window
+@vindex window-group-start-function
+This function is like @code{window-start}, except that when
+@var{window} is a part of a group of windows (@pxref{Window Group}),
+@code{window-group-start} returns the start position of the entire
+group.  This condition holds when the buffer local variable
+@code{window-group-start-function} is set to a function.  In this
+case, @code{window-group-start} calls the function with the single
+argument @var{window}, then returns its result.
+@end defun
+
 @cindex window end position
 @defun window-end &optional window update
 This function returns the position where display of its buffer ends in
@@ -3124,6 +3159,18 @@ way real redisplay would do.  It does not alter the
 text will end if scrolling is not required.
 @end defun
 
+@vindex window-group-end-function
+@defun window-group-end window update
+This function is like @code{window-end}, except that when @var{window}
+is a part of a group of windows (@pxref{Window Group}),
+@code{window-group-end} returns the end position of the entire group.
+This condition holds when the buffer local variable
+@code{window-group-end-function} is set to a function.  In this case,
+@code{window-group-end} calls the function with the two arguments
+@var{window} and @var{update}, then returns its result.  The argument
+@var{update} has the same meaning as in @code{window-end}.
+@end defun
+
 @defun set-window-start window position &optional noforce
 This function sets the display-start position of @var{window} to
 @var{position} in @var{window}'s buffer.  It returns @var{position}.
@@ -3187,6 +3234,19 @@ off screen at the next redisplay, then redisplay computes a new window-start
 position that works well with point, and thus @var{position} is not used.
 @end defun
 
+@vindex set-window-group-start-function
+@defun set-window-group-start window position &optional noforce
+This function is like @code{set-window-start}, except that when
+@var{window} is a part of a group of windows (@pxref{Window Group}),
+@code{set-window-group-start} sets the start position of the entire
+group.  This condition holds when the buffer local variable
+@code{set-window-group-start-function} is set to a function.  In this
+case, @code{set-window-group-start} calls the function with the three
+arguments @var{window}, @var{position}, and @var{noforce}, then
+returns its result.  The arguments @var{position} and @var{noforce} in
+this function have the same meaning as in @code{set-window-start}.
+@end defun
+
 @defun pos-visible-in-window-p &optional position window partially
 This function returns non-@code{nil} if @var{position} is within the
 range of text currently visible on the screen in @var{window}.  It
@@ -3228,6 +3288,21 @@ Here is an example:
 @end example
 @end defun
 
+@vindex pos-visible-in-window-group-p-function
+@defun pos-visible-in-window-group-p &optional position window partially
+This function is like @code{pos-visible-in-window-p}, except that when
+@var{window} is a part of a group of windows (@pxref{Window Group}),
+@code{pos-visible-in-window-group-p} tests the visibility of @var{pos}
+in the entire group, not just in the single @var{window}.  This
+condition holds when the buffer local variable
+@code{pos-visible-in-window-group-p-function} is set to a function.
+In this case @code{pos-visible-in-window-group-p} calls the function
+with the three arguments @var{position}, @var{window}, and
+@var{partially}, then returns its result.  The arguments
+@var{position} and @var{partially} have the same meaning as in
+@code{pos-visible-in-window-p}.
+@end defun
+
 @defun window-line-height &optional line window
 This function returns the height of text line @var{line} in
 @var{window}.  If @var{line} is one of @code{header-line} or
@@ -3471,6 +3546,19 @@ the top of the window.  The command @code{recenter-top-bottom} offers
 a more convenient way to achieve this.
 @end deffn
 
+@vindex recenter-window-group-function
+@defun recenter-window-group &optional count
+This function is like @code{recenter}, except that when the selected
+window is part of a group of windows (@pxref{Window Group}),
+@code{recenter-window-group} scrolls the entire group.  This condition
+holds when the buffer local variable
+@code{recenter-window-group-function} is set to a function.  In this
+case, @code{recenter-window-group} calls the function with the
+argument @var{count}, then returns its result.  The argument
+@var{count} has the same meaning as in @code{recenter}, but with
+respect to the entire window group.
+@end defun
+
 @defopt recenter-redisplay
 If this variable is non-@code{nil}, calling @code{recenter} with a
 @code{nil} argument redraws the frame.  The default value is