fix nasty generation bug

now, generate() will never populate the 3x3 block starting at (6,6)
before calling resolve(), because if it did
resolve would go totally crazy and run for a very long time
(I have never found out how long. Normally, on my machine,
total run time of the test program is a few milliseconds.
When resolve() goes crazy, it spun around for minutes without a result
so I always killed it.)