arbitrary unclaimed square. This cures the most common cause of
generation failures (covering a large area in dominoes was the most
difficult case, and would fail even if the large area was 1xn!); the
failure rate is now sufficiently low under all circumstances I've
found that I'm willing to just loop until I get a success.
[originally from svn r7693]
rectangle into equally sized ominoes. I have a couple of potential
applications for this, but none I've actually implemented yet, so
for the moment it's living in `unfinished'.
[originally from svn r7690]
