The Free On-line Dictionary of Computing (30 December 2018):

Xemacs
Lucid Emacs

    (Originally "Lucid Emacs") A text editor for the
   X Window System, based on GNU Emacs version 19, produced
   by a collaboration of Lucid, Inc., SunPro (a division of
   Sun Microsystems, Inc.), and the University of Illinois.

   Lucid chose to build part of Energize, their C/C++
   development environment on top of GNU Emacs.  Though their
   product is commercial, the work on GNU Emacs is free
   software, and is useful without having to purchase the
   product.  They needed a version of Emacs with mouse-sensitive
   regions, multiple fonts, the ability to mark sections of a
   buffer as read-only, the ability to detect which parts of a
   buffer has been modified, and many other features.

   The existing version of Epoch was not sufficient; it did not
   allow arbitrary pixmaps and icons in buffers, "undo" did
   not restore changes to regions, regions did not overlap and
   merge their attributes.  Lucid spent some time in 1990 working
   on Epoch but later decided that their efforts would be better
   spent improving Emacs 19 instead.

   Lucid did not have time to get their changes accepted by the
   FSF so they released Lucid Emacs as a forked branch of
   Emacs.  Roughly a year after Lucid Emacs 19.0 was released, a
   beta version of the FSF branch of Emacs 19 was released.
   Lucid continued to develop and support Lucid Emacs, merging in
   bug fixes and new features from the FSF branch as appropriate.

   A compatibility package was planned to allow Epoch 4 code to
   run in Lemacs with little or no change.  (As of 19.8, Lucid
   Emacs ran a descendant of the Epoch redisplay engine.)

   [Update?]

   (2000-05-16)