RE: how to highlight fine differences in Diff mode?

[Drew Adams]

    > The library I mentioned was diff-mode-.el, not diff-mode.el.
    > The former is a minor enhancement to the latter. The latter
    > has recently been updated in CVS, and is now very similar to
    > the former. If you are using CVS Emacs, you can use just the
    > latter. If you are using Emacs 21, you might want to use
    > both.

    Thanks so much for the explanation!  Makes it all clear.

    Maybe edit that (excellent) explanation into the file (if it's
    not already there -- haven't yet grabbed diff-mode-.el).

Yes, there is such an explanation in the file.

There is also a more general explanation here:
http://www.emacswiki.org/cgi-bin/wiki/DrewsElispLibraries#LibraryNames.