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The Crab is a kind of `restricted knight', introduced by Ralph Betza in 1996. The name
clear from the diagram: the squares the crab can go to are like the pincers of a crab.
The crab moves like a knight, jumping across squares, but only in some directions: it can go either two squares
vertically forwards and one square horizontally, or one square vertically backwards and two squares horizontally.
The crab can jump to all the squares marked with a circle.