Black's a2 pawn is about to queen. But it is White to play now. What should be White's best play?
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1. Bh7+ Kh8 - How should White continue?
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2. Rd1 b3 - the bishop on h7 is en prise, and Black is also threatening to push b3-b2. What is White's only move to keep the advantage?
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3. Kb4 b2 - How should White continue? There is no stopping Black from queening one of the pawns! (Note that 3. Kb4 Kxh7 can be met with 3...Kc3 now, which stops the pawns definitively!)
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4. Bb1 a1=Q - Black has queened. What is White's winning move? This is the final move of the exercise!