A problem which adds an original twist to a well-known idea. It requires the solver to find a thematic try as well as the correct key move.
The thematic try 1.Qc1 threatens 2.Qf4. Knight defences on c4 are met by knight mates on d4:
1...Sdc4 2.Sfxd4;
1...Sac4 2.Sexd4;
(also 1...Qb3 2.Sexd4).
The refutation is 1...Qc4!
The key, 1.Re1, threatens 2.Re5. The knight defences on c4 still work, but the mates are now reversed.
1...Sdc4 2.Sexd4;
1...Sac4 2.Sfxd4;
(also 1...Se4 2.dxe4 and 1...Qa5 or b5 2.Sfxd4).
Reciprocal change of mates has been a standard theme for many decades, but the use of only two thematic squares and one type of piece makes this one of the most original examples, and shows great ingenuity on the part of the composer. The g2 pawn prevents a cook by 1.Sh2+ Ke5 2.Sg4, while the black bishop prevents a cook by 1.Sfxd4+ Ke5 2.Re1. Marjan later decided that a version with the black bishop moved to g6, black pawns at h7 and h5, and the g2 pawn removed, is an improvement, as the key piece does not stand en prise.
Peter Niehoff: A surprising reciprocal change with the two thematic defensive moves to the same square and likewise for the mates. A fantastic and difficult mechanism.
Dafydd Johnston: Abandoning the battery is counter-intuitive, and the solver could easily fall for the tempting 1.Qc1.