Change gnus-set-difference from O(N^2) to O(N)

[gnus] / todo

;; Also know as the "wish list".  Some are done. For the others, no
;; promise when to be implemented.

* Use a new custom type (`define-widget') for posting-style in `gnus-cus.el'
  (G c) and for `gnus-posting-styles'.  Maybe some allowed types are still
  missing.

* Add proper doc strings to functions and variables explained in the manual
  (info "(gnus)Gnus Utility Functions")

* Add Message-IDs or URLs refering to relevant discussions on lists and
  newsgroups.

* Use nicer tool bar icons from GNOME

  Done for Emacs (The GNOME icons won't fit into standard XEmacs icons,
  IMHO. -- rsteib) in group, summary and message mode.

  Some modes might also deserve improved tool bars:

  - gnus-draft-mode

  - mml-preview buffer:

    . zap most buttons; except print, customize (?) and help

    . "exit" should just kill the buffer

  - gnus-server-mode: Add some commands from the Connections and Server