bug#16330: 24.3; wish: after hitting a prefix-key, show possible options

[Drew Adams <drew.adams@oracle.com>]

> I often find myself in a position where I still know some prefix
> key, but not the second key. For example I hit `C-c /` in mailcrypt-
> mode, but I'm not sure whether I fetch the key with `k` or with `f`
> (the latter actually calls forget passphrase). In that case, I am
> forced to either consult C-h m or use the menu - both of which does
> not feel very elegant.
> 
> Instead of that, it would be nice if emacs could show a completion-
> list similar to the function completion with M-x. This would allow
> discovering options while using emacs instead of browsing menus or
> tutorials, hunting for new ways of working more efficiently.

1. You can hit `C-h' to see the list of keys that start with that
prefix key.  E.g., `C-c / C-h' shows you all of the keys that start
with prefix key `C-c /'.

2. If you use Icicles then you can hit `S-TAB' to complete the
prefix key.  Buffer *Completions* shows the possible completions.
Type text matching the rest of the key name to finish the key.

http://www.emacswiki.org/emacs/Icicles_-_Key_Completion

#1 is available in vanilla Emacs.  It might well be all you need.

#2 requires Icicles.  It corresponds more exactly to what you
requested, IIUC.