RE: ffap bindings suggestion

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

      In one of my libraries, I use a minor mode, and restore the
      original bindings when the mode is exited. I find that
      clean. However, the library currently just restores the vanilla
      Emacs (-q) bindings; it would of course be better to save the
      bindings at the time of entry into the mode, and restore those
      (original user bindings) when the mode is exited. I'm not
      referring here to a keymap that is local to the mode; in my
      case, the minor mode changes minibuffer key bindings. (Using a
      local keymap is obviously the way to go when appropriate.)

      Is there no recommended way (or recommended ways) to handle
      this? It's not uncommon for a library to let users adopt the
      library's suggested (multiple, often global) bindings in some
      easy way. Perhaps we should come up with a recommended way for
      libraries to do that. That way should, in the best case, let
      users get back their original bindings when they no longer want
      to use the features of the library.

    No ideas/opinions on this?

  Too general for me to answer.  I'm not really sure what you're
  talking about.  Make it more concrete.

When minor mode is turned on:

  (substitute-key-definition 'scroll-up 'foobar-cmd
                             minibuffer-local-completion-map
                             global-map)

When it is turned off:

  (substitute-key-definition 'scroll-up nil
                             minibuffer-local-completion-map
                             global-map)

That's what I do: bind keys that are bound globally to `scroll-up' to
`foobar-cmd' in minibuffer-local-completion-map. Then restore those
keys to their original bindings in minibuffer-local-completion-map.

I'm asking for a recommended way to accomplish this.

  I think you've already seen here some example code that modifies a
  keymap and then restores the original one, using inheritance (so the
  "save" and the "restore" of changed key-bindings are done in one
  step each).
    
Could you please point to that information (discussion). I searched
for "inherit" + "map", but I didn't notice it.

Again, the need is to modify an existing keymap (such as
minibuffer-local-completion-map), not to use another keymap. Existing
code expects to use the existing map.