[14+10. Weiß nimmt den letzten Zug zurück; dann Matt in einem Zug]
White must retract -1. Bh1-a8! and then play
Since wBc1 was captured at home, the wBg7 must be a promoted bishop.
The Wh. a-pawn must be the one that has promoted, since none of the
other pawns currently on the board could be the pawn from a2. Since
the promotion took place on a black square, the pawn couldn't have promoted
on b8 or d8, since the bishop could not have escaped. Therefore it was
f8 (can't be h8 since the wPa2 can't get there.) Therefore the wPa2
All these captures took place on light squares; therefore, the black
KB was not captured by the wPa2, so that the wPa2 captured 2 bRs, 2
bNs, and the bQ. When e6xf7 was played, Black had already played f7-f6
but not yet d7xe6. Therefore, in order for the bRa8 to escape to be
captured by the wPa2, b7-b6 must have been played (so that the bBc8
could get out, in turn freeing the bRa8). So both of the pawn advances,
b7-b6 and f7-f6, occurred before the promotion.
Therefore, whatever White's last move was, Black's move before was
not b7-b6 or f7-f6. If we try -1. N-d7 d7xQe6 -2. Q-e6+, Black is retropat.
Therefore, Black's last move was not with any of his pawns. Can White
uncapture the Black KB to allow Black to move it? No, because the only
White pieces on dark squares that could have made this capture are wNf8
and wRh4, and both squares leave the resulting KB with no move and thus
Bl. would be retropat. Therefore Black's last move was with his king
No matter what White's retraction is, the Black king will be in double
check after Black retracts Ke4-f5. The only White retraction leading
to a legal double check is -1. Bh1-a8! Ke4-f5 -2. Rg2-e2++ Kf3-e4+,
so White's last move was -1. Bh1-a8.
Solution by Philippe Schnoebelen