(solving it only requires matrix inversion over GF(2), whereas
several of the other puzzles in this collection are NP-complete in
principle), but it's a fun enough thing to play with and is
non-trivial to do in your head - especially on the hardest preset.
[originally from svn r5967]
binary if we choose: fix bugs in cube.c and sixteen.c that manifest
when compiled that way, and introduce list.c which provides a global
list of all the available puzzles.
[originally from svn r5169]
