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Of all the forms of chess problems, I find retrograde problems the most interesting. In a 'retro' you
have to work backwards, to work out what the last move was, or whether a promoted piece was on the board etc Raymond Smullyan, mathematician, logician and magician, is one of the leading composers of these type of problems, and the diagrammed position is due to him. As retrograde problems go, it is fairly easy, but there is still a delight in finding the answer. While it is White to move, the problem is find the last 2 moves (by black and white) that lead to this position.