Limit what M-x TAB lists?

["Stephen J. Turnbull" <stephen@xemacs.org>]

Lars Ingebrigtsen writes:
 > Here's today's unrealistic idea: Wouldn't it be nice if `M-x fooTAB' was
 > more useful?
 > 
 > For instance, if I type `M-x vc-dTAB' in this buffer, I get the
 > following: 
 > 
 > Possible completions are:
 > vc-default-check-headers 	vc-default-mark-resolved
 > vc-default-revision-completion-table 	vc-delete-file
 > vc-diff 	vc-dir
 > vc-dir-delete-file 	vc-dir-find-file
 > vc-dir-find-file-other-window 	vc-dir-hide-state
 > vc-dir-hide-up-to-date 	vc-dir-ignore
 > vc-dir-isearch 	vc-dir-isearch-regexp
 > vc-dir-kill-dir-status-process 	vc-dir-kill-line
 > vc-dir-mark 	vc-dir-mark-all-files
 > vc-dir-menu 	vc-dir-mode
 > vc-dir-next-directory 	vc-dir-next-line
 > vc-dir-previous-directory 	vc-dir-previous-line
 > vc-dir-query-replace-regexp 	vc-dir-refresh
 > vc-dir-search 	vc-dir-show-fileentry
 > vc-dir-toggle-mark 	vc-dir-unmark
 > vc-dir-unmark-all-files 	vc-dir-unmark-file-up

vc-default-...
vc-delete-file
vc-diff
vc-dir
vc-dir-...

is simple to compute algorithmically (string processing).  It doesn't
do what you want, obviously, but it sure reduces the visual clutter.

Another algorithmic approach would be to mark each symbol defined in a
keymap with the property 'completion-keymap and the keymap (list of
keymaps).  If none of the keymaps in which a symbol is defined are
currently active, filter the symbol out.  Heck, maybe the property is
a premature optimization; perhaps which keymaps bind the function and
whether they are active can be done sufficiently quickly.