* lisp/mail/rmailsum.el (rmail-summary-by-thread): Prefix unused
lambda args with '_'.

* lisp/mail/rmailsum.el (rmail-summary-invert):
(rmail-summary-direct-descendants):
(rmail-summary--walk-thread-message-recursively):
(rmail-summary-by-thread):
(rmail-summary-by-labels):
(rmail-summary-by-recipients):
(rmail-summary-by-regexp):
(rmail-summary-by-topic):
(rmail-summary-by-senders): Use the boolean values in
'rmail-summary-currently-displayed-msgs' and similar directly instead
of comparing them to t and nil.

* lisp/mail/rmailsum.el (rmail-summary--walk-thread-message-recursively):
Use `mapc' instead of `mapcar'; we don't care about the results.

* lisp/mail/rmailsum.el (rmail-summary-subjects-hash-table)
(rmail-summary-message-parents-vector)
(rmail-summary-message-ids-hash-table): New variables.
(rmail-summary-fill-message-ids-hash-table)
(rmail-summary--split-header-field)
(rmail-summary-fill-message-parents-vector)
(rmail-summary-direct-descendants)
(rmail-summary--walk-thread-message-recursively)
(rmail-summary-by-thread): New functions.

* etc/NEWS: Announce the new Rmail features.

* lisp/mail/rmailsum.el (rmail-summary-progressively-narrow):
Renamed from 'rmail-summary-apply-filters-consecutively'.  Doc
fix.
(rmail-summary-invert): Renamed from 'rmail-summary-negate'.  Doc
fix.
(rmail-summary-currently-displayed-msgs): Doc fix.
(rmail-summary--exists-1): New function.
(rmail-summary-by-labels, rmail-summary-by-recipients)
(rmail-summary-by-regexp, rmail-summary-by-topic)
(rmail-summary-by-senders): Call 'rmail-summary-exists' to verify
that the summary is usable.

* lisp/apropos.el:
* lisp/mail/feedmail.el:
* lisp/mail/mail-hist.el:
* lisp/org/org-ctags.el:
* lisp/org/org-protocol.el:
* lisp/progmodes/ebnf2ps.el:
* lisp/progmodes/mixal-mode.el:
* lisp/progmodes/perl-mode.el:
* lisp/savehist.el:
* lisp/textmodes/table.el:
* lisp/vc/smerge-mode.el: Remove redundant installation instructions
for packages bundled with Emacs.