I recall playing with this on the -11, but it seems to
have become extinct
(the program, I mean). I seem to recall that it was written in PDP-11
assembly; did it ever get rewritten in C?
Looks like rewritten at least twice (1983 and 1990):
http://svnweb.freebsd.org/csrg/games/factor/
Also a maintained version
http://cvsweb.netbsd.org/bsdweb.cgi/src/games/factor/