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Retractor (19 Feb 2003 at 00:26)
Retractor
Here's a fun chess puzzle I found. It's a retrograde analysis puzzle (my favorite kind), meaning that you have to play chess in reverse for part of it. The specification here is for white to retract one and then mate in one. That is, white must take back one move that he may have just legally made -- and then make a different move from that position to mate black.

This problem would be unremarkable except that it is a problem four times over: once you've solved the puzzle as pictured, solve it again with all of the pieces shifted up one space. The solution will be completely different, and again there will be only one way to do it. Then, solve with the pieces shifted up another space on the board -- again, a different solution. Finally, solve it a fourth time with the pieces up a final space. Minimal and clever!

Hint: Note that you can take back captures, inventing the presence of any black piece that could legally have been captured by white. Also, you should brush up on your obscure chess moves if you're not entirely confident. Since each position will have a different solution, the solutions will of course be position-dependent; since a pawn can do different things based on where he is, it's all going to be about those pawns.
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