🚀 go-pugleaf

In article <JASON.93Aug3133725@cygnus.cygnus.com> jason@cygnus.com (Jason Merrill) writes:

   The only problem I've had with the Emacs 19-patched version of 5.33 has
   been that vm-edit breaks when trying to construct the keymap.  Since v19
   has a convenient keymap inheritance scheme, I've patched vm-edit.el to use
   that instead of vm-overlay-keymap.  Note that this code will need to be
   massaged in order to work with v18 as well.

   [...]

   ! 	    (use-local-map (append vm-edit-message-mode-map
   ! 				       (copy-keymap (or (current-local-map)
   ! 					    (make-sparse-keymap)))))
   ! ;	    (vm-overlay-keymap vm-edit-message-mode-map (current-local-map))

Yep.  "append" is sure what I'd call a "convenient" (along with
"obvious", "clear", "transparent" and "abstract", amonst others) name
for a function for modifying the structure of the keymap datatype.

How about
(cond ((fboundp 'set-keymap-parent)
       (defun vm-overlay-keymap (src dest)
         (let ((map (copy-keymap src)))
           (set-keymap-parent map dest)
           map)))
      ((string-match "\\`18\\." emacs-version)
       (defun vm-overlay-keymap (src-map dest-map)
         (setq dest-map (copy-keymap dest-map))
         ...horrendous old definition from VM...
         dest-map))
      (t
       (defun vm-overlay-keymap (rms loses)
         (append rms loses))))

and then just

	    (use-local-map (if (current-local-map)
                               (vm-overlay-keymap vm-edit-message-mode-map
                                                  (current-local-map))
                               vm-message-mode-map))