"The second problem," continued Holmes, "is relatively easy. What piece was captured by the White pawn on a3? Not the bishop from f8, which never escaped from its home square; not the bishop from c8, which travels on white squares. The third missing Black piece is the pawn from h7. But this, of course, could not have gotten to a3. Hence the pawn from h7 has promoted. Now, this pawn crossed the second row (from White's side) neither on square f2 nor on d2(since the White king never moved) but on e2. Thus the pawn made two captures to get to f3 (after the pawn from e2 moved to e3 and before the White pawn on f3 moved from f2), then one capture on e2, and then its fourth capture on either d1 or f1 Since the pawn on f3 did not move till after the Black pawn was on e2, the White king's rook could not have yet got out to be captured. Therefore the rook was the last White piece to fall, and hence was captured on d1 or f1. But as long as the Black pawn was on e2,: this blocked the White rook from getting to d1. Hence the rook fell on f1."
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