In this remarkable little problem by Jean-Michel Trillon, you cannot possibly understand what the composer was trying to do until you have solved the whole thing. It is a help-mate in five, which means that Black moves first, and the players co-operate to reach a position in which White gives mate on his fifth move.

The plot is not too difficult to find: White is going to have to get his king to a4, then play Nc3 mate, which means that his first four moves must be Kd7, Kxc6, Kb5 and Ka4 - quite a minefield to cross when every square is protected.

Black's moves must therefore be: 1. Rook gets off the seventh rank; 2. Other rook moves to b7;

3. Rook gets off the b-file;

4 and 5. A rook gets back to b2. And Black must also ensure that the line of the bishop on h8 to c3 is blocked.

The only way to do it is like this: 1.Rg8+ Kd7 2.Rb7+ Kxc6 3.Rbg7 (getting in the bishop's way in good time) Kb5 4.Rb8+ (the only way back to b2) Ka4 5.Rb2 Nc3 mate.

And now you can understand what the composer was trying to do: he had set himself the task of swapping the two black rooks around. A very witty composition and a pleasure to solve.