Does anyone have some hard (long-running) examples? I have a collection but only one seems to take any appreciable time. Also have any of you played with under-constrained puzzles (lots of wide open spaces), and if so have you need to develop any special heuristics to avoid an extensive search in the vast empty spaces? What about dead-end detection (you've closed off a section of the puzzle and it is thus unsolvable until you backtrack, assuming you have a search-tree-like algorithm)? I've attached a 15x15 that is reasonably hard, but still not enough to take a long time.
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