Solution

**Laurent Joudon**

Rex Multiplex, 1983

4+3. (Retro-Volages) Mate in 2

The wRh4 cannot be the original wRa1 because it could not have reached h4 without visiting a light square. But it can be a promoted wP so that, by default, it must be assumed volage. The wNa6 is not volage anymore.

Mate in 2 starts with **1. Rb4 !!**, threatening 2. Rb8#.

There the wR remains on dark squares, so that the volage assumption does not trouble the play.

Black only defense is **1 ... O-O**. But is castling
allowed? Black castling is legal if both bKe8 and bRh8 never moved.
So what is Black last move? With the volage rule, Black's last move
could have been -1. Nb8-a6[=wN] or -1. R~-h4[=wR].

If we assume -1. Rh4[=wR], we know that the wRh4 is not volage anymore. If we assume -1. Nb8-a6[=wN], we know the wRh4 cannot have been promoted on b8. And a promotion on d8 or f8 or h8 would have broken Black's O-O.

In all cases, after 1 ... O-O, we know the wR is not volage anymore.
So that **2. Rg4!!#** mates.