Udiknedormin, thank you for the very detailed and helpful answer!
I changed flip and transpose per your suggestion and transform is now just:
iterator transform(s: var seq[seq[bool]]): seq[seq[bool]] =
for _ in 1 .. 4:
yield s; s.transpose
yield s; s.flip
Nicer looking code for sure, but this was/is just a small minority of time
spent, so speed improvements aren't that great (barely noticeable).
.
> I think it might be nice to give 2D array a try.
I'll try that next to see if it gives any speedup to expand, findNew9x9, and
findNext.
