bad definition for `function' widget

[Dave Love]

If browse-url-browser-function is an alist, the customization buffer
for it matches it as a function:

  Browse Url Browser Function: Hide Value Value Menu Your own function: 
  (("^mailto:"; . browse-url-mail)
   (".*" lambda
    (url &optional new-window)
    (funcall
     (if window-system 'browse-url-galeon 'browse-url-lynx-emacs)
     url)))
     State: this option has been set and saved.

and you can't save an edited version.

It's because the widget `function' inherits from `sexp' when it should
be `restricted-sexp' to make :match-alternatives work.  (It looks as
though I failed to fix it properly before...)

Also the `:prompt-match' slot clearly shouldn't be `fboundp' for a
general function, but I don't know whether the :prompt-... slots
should be defined at all.  (It would be good if widget slots were
documented generally.)