A trifle, really, compared to the last two. 1 a8(B)! is the key move. Now 1...Ke8 2 Ke6 or 1...Kg8 2 Kg6 will mate with 3 b8(Q), but the point of the text move is 1...Kf8 2 b8(Q)+ Kf7 3 Bd5 mate.
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