V.Onitiu : White retracts a move and mates in 1. White missing pcs: Bc1 = 1 Black missing pcs:  Q,N,N,Bc8,Pa,Pc,Pe = 7 Captures:  Black's only capture is evident  h6xBg5. White  has captured Bc8 with a piece and the white pawn captures have been b2xc3, c2xd3 and exf . All these pawns have captured a white piece, since black can not have more doubled pawns. Capturing the c-pawn with b2xc3 would prevent the promotion of  'extra' bishop as analysed  below. So captures of black Q,N,N and Bc8 are explained. This leaves the three pawns. We know that at least one of them is promoted to  Bg3 or Bg1. Other two may have been captured by yet unspecified piece.

The Analysis: It is clear from the diagram that whichever retraction is done, the only possible mating move for white is h5xg6 ep #.
The convention for chess compositions is that the en passant capture is justified as the key move only it it can be proven that the
preceeding move a double step of the opponent's  relevant pawn . We need to retract a white move that proves g7-g5 as black's last.

This can only be done if we retract o-o-o! After this crafty retraction several new aspects become clear:
- The Ke1 and Ra1 cannot move any more in further retractions. King also blocks black e-pawn, which never promoted.
- The capture e3xf4 would now set up an unbreakable prison for Rf2. Therefore this capture was e2xf3, allowing rook to return via f3.
- Before the unlocking e2xf3 capture can be retracted, The Bh7 must be reinstalled on f1.
-The black a- and e-pawns have not promoted, but they are missing. Therefore they were captured on their own files. by a white piece.
- The extra bishop is thus promoted from the black c-pawn.  However, before the promotion Bc1 must move away, which can  only
happen after the b2xc3 capture (which blocks the c-file). Therefore the order of events must have been as follows:
* White captures c2xd3.
* Black c-pawn marches to c2.
* White captures b2xc3.
* White Bc1 goes up to g5 and is later captured by  h6xg5.
* Black promotes c2-c1=B and takes this bishop to g1 or g3.
* White locks the  bishops and Rf2 in the cage with  e2xf3.
* White takes the released Bf1 up to  h7.

The retractions must happen in the reverse order to above. Therefore getting Bh7 back to f1 is the first priority. However, after  retracting
g7-g5, black is very short of further available retractions (in fact there is only  g5-g4).  White must immediately provide more moving men
for black. And only e- and a-pawns can be uncaptured before bishop is safely on f1. Fact is that the initial series of retractions is forced:
-1. U-U-U g5-g7 -2.Bh8-e4 g4-g5 -3.Be4-c6(xP) e4-e5 -4.Bc6-a4 e5-e6 -5.Ba4-d1(xP) a4-a5 -6.Bd1-e2 a5-a6 -7.Be2-f1 a6-a7 -8.f3-e2(xQ) Qf3~
and then the position unfolds. You can follow a complete retrogame (without comments) here

One could ask why not g6-g5 instead of g7-g5 as the last move? But in that case -1.U-U-U g5-g6 -2. Bh7-g8 (The only one!) g6-g7 -3.Bg8-h7
with the same position as before, but one -crucial- tempo late. Also trying to create black pawns with  Rh6 will soon lead to a retrostalemate.
So indeed g7-g5 is the only legal move preceeding white's O-O-O. Retracting anything different  from uncastle can not justify the en passant.

The solution is: White retracts long castling and instead mates with h5xg6 ep #.    Back