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A witty idea. Clearly the rook will mate, but White must be careful to avoid
Solution: 1.a8S. A strange looking promotion, but the white king must
be able to escape checks without blocking the b-file. 1...Sc5+ 2.Ka7 Sb7
and now the point of White's promotion is revealed. 3.Sc7! Had White
chosen any other promotion he would be in zugzwang, but the knight is able
to play a waiting move, after which Black's knight must vacate the file,
allowing 4.Rb1 mate. Had Black played 2...Sb3 3.Rb4 would mate next move.
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