[9+11. Proca-retractor. Weiß nimmt zwei Züge zurück; dann Matt in einem Zug]
White retracts -1. O-O! forcing -1...O-O-O -2. b4-b5
and then 1. Rh1-h8#.
Why are -1...O-O-O and -2. b4-b5 forced?
White captures are axb, g2xf3xe4xd5xc6. Since the wPg2 captured on
light squares only, the black KB must have been captured by the a-pawn,
so axb must be a3xb4. Black captures are d7xe6, g7xf6, h7xg6xf5xe4xd3xc2.
These captures account for all missing pieces.
To extricate bPc2, we must undo d2-d3. Before that, we must bring
the Wh. QB back to c1. Wh. QB must have been captured on f6. To undo
g7xBf6 we must first undo a3xBb4. But in order to avoid locking out
the white QR (the Wh. king can't let it back in "after" -1 O-O), we
must undo d7xRe6, and before that we must uncapture the black QB along
the sequence of captures by the wPg2 and bring it back to c8, and we
must bring the Bl. QR back to a8 or b8. To reiterate, the sequence of
events was: (1) d3xc2, (2) d2-d3, (3) wBc1-...-f6 and g7xBf6, (4) bBf8-...-b4
and a3xBb4, (5) wRa1-...-e6 and d7xRe6. Only then did the Bl. QR and
Now "after" -1 O-O!, Wh. has only 4 moves, b4-b5 and 3 uncaptures
with the wPc6 (it can't get all the way back to g2 without locking out
the wBf1) before Bl. must uncapture d7xRe6 to avoid Wh. retropat, so
we must conduct the bQR and bQB back to a8/b8 and c8, respectively,
within that time. The only way this can be done is with -1...O-O-O -2.
b4-b5 Qd8-d4 -3. d5xBc6 Bd7-c6 -4. e4xd5 Bc8-d7 -5. f3xe4 d7xRe6. (-2.
d5xBc6? is premature; it blocks the Bl. Q's route back to d8. After
-1...O-O-O, Bl. Q must also go back to d8 before d7xRe6 can be undone.)
Solution by Philippe Schnoebelen