This is how defcustom can look like at worst !

gistfile1.el

```emacslisp
(defcustom twittering-edit-skeleton 'none
  "*A symbol specifying an effective skeleton.
It must be one of a symbol in `twittering-edit-skeleton-alist'.
When entering `twittering-edit-mode', the skeletons in the specified
entry in `twittering-edit-skeleton-alist' are performed."
  :group 'twittering-mode
  :type (if (> (length (mapcar #'car twittering-edit-skeleton-alist)) 0)
	    `(choice ,@(mapcar (lambda (s) `(const ,s))
			       (mapcar #'car twittering-edit-skeleton-alist)))
	  'symbol))
```