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Oof, really really clever stuff. You've got one of those core mechanics that's so simple to understand (so much so that you can fit that nice numbered grid in the top right corner!), but has so much depth to be teased out of it. I think an undo button would have been a really nice touch to let the player fully appreciate your puzzles and learn from their mistakes, but hey - you've given us more than enough to appreciate as is!