[14+11. (AP) Matt in 4 Zügen]
Suppose white can castle. White captures were hxg, gxf, fxe, exd and
c6xb7. Black captures were dxcxd (to let the white d-pawn pass without
capture). So black's last move wasn't a capture. The only move that
will give white a move that'll leave black with a new retro-move is
c7-c5 (and white's last move was c6xb7). So if white may castle, black
played c7-c5 as last move.
1. dc6! d3 2. OOO! (forced to prove the legality of the en passant capture)
d2 3. Rd2 Be6 4.d7#.
Solution by Philippe Schnoebelen
