Mate in 4

Thematic tries highlight the weaknesses in Black’s post-key defences: 1.Rb6? (threat 2.Bxb5) Sa3 (prevents Ba3) 2.Rc6 (threat 3.Rxc5) but 2…Rxa2! 1.Rc6? (threat 2.Rxc5) Ba3 (prevents Sa3) 2.Rb6 (threat 3.Bxb5) but 2…Rb1! also 1.Kb6? (threat 2.Bxb5) Sa3! The key 1.Ra6 guards the a-file and threatens 2.Se5+ Kb4 3.Sfd3 or Sxd5. Black can defend by guarding e5. 1...Rg5 (now Rxa2 is impossible) 2.Rb6 (threat 3.Bxb5) Sa3 3.Rc6 and 4.Rxc5; 1...Rh5 (now Rb1 is impossible) 2.Rc6 (threat 3.Rxc5) Ba3 3.Rb6 and 4.Bxb5; 1...f6 2.Kb6 (threat 3.Bxb5) Sa3 3.Bxb5+ Sxb5 4.Ra4. A highly-unified problem with both black rooks operating on rank and file, and mutual obstructions at a3.

Guy Meissonnier: A difficult four-mover! Nice rook swallow-moves.