[I searched the archives, but didn't find a previous discussion about this; if I missed it, please just point me in the right direction. Thanks.]
I've been using both XEmacs(20) and Emacs(20), and while investigating some of their differences in behavior I stumbled upon http://www.xemacs.org/About/XEmacsVsGNUemacs.html which quotes RMS as having written (no date was specified): ... But in another sense it is not GNU software, because we can't use XEmacs in the GNU system: using it would mean paying a price in terms of our ability to enforce the GPL. Some of the people who have worked on XEmacs have not provided, and have not asked other contributors to provide, the legal papers to help us enforce the GPL. I have managed to get legal papers for some parts myself, but most of the XEmacs developers have not helped me get them. ... But this is worse than competition--it is unfair competition. The XEmacs developers can and do copy code they like from Emacs. If I could copy the code I like from XEmacs in the same way, at least the rivalry would be fair. But I can't do that't, because substantial parts of XEmacs don't have legal papers, or don't have known authors. ... Is that still an accurate description of the legal status (from FSF's perspective) of XEmacs, and if so, shouldn't we move it to non-free?