[15+10. (RV) Matt in 3 Zügen]
Suppose black can castle both king- and queenside. Then his last move
was either b7-b5, d7-d5 or f7-f5.
- Last move was b7-b5: #3 with 1. cb6 Ra4 2.Rh8 Qh8 3. Qh8# 1. ..
Ke7 2. Bb4 Kd8 3. Ra8#.
- Last move was d7-d5: #3 with 1. cd6, and black can't defend against
2. Rh8 Qh8 3. Qh8#.
- Last move was f7-f5: #3 with 1. gf6, and black can't defend against
the two threats 2. Rh8 Qh8 3. Qh8#, and Ra8#.
Suppose now black can't castle queenside. Then #3 with 1. Bd6!, and
(again) black has no proper defense against 2. Rh8 or 2. Ra8.
And the last case: Black can't castle kingside. Then #3 with 1. Bf6!
Solution by Philippe Schnoebelen
