Re: Undo discard prompt

[Richard Stallman]

      Now I can
    also undo it by using diff-unified->context, so if I could replace
    the large undo entries with just one entry that says "call
    diff-unified->context" it would not be a problem any more.

The idea of extensible undo entries is interesting, but using it for
composite operations such as this would be unreliable.  It is better
to undo them based on their actual edits.

                                                                The SES
    spreadsheet also needs such a feature (and currently fakes it by using
    a wrapper around primitive-undo).

This seems like the right sort use for such a feature.

However, the use of a wrapper around primitive-undo is very ugly.
I think we need to do more to detect when such things are installed
in Emacs, so we can systematically make sure they are not.