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Before setting our Christmas competition three weeks ago, I gave it to two passing grandmasters to test. Two days later, neither had found the solution, and one of them has not spoken to me since.

Congratulations, therefore, to all 28 readers who sent in correct answers. For the grandmasters, and anyone else still struggling with the position, here is an explanation.

g, ,a,f, n nhnhc ,f. .h, , , , n , , , , N , , ,G HCdNHNHN ,sZ ,DVG

The task, when stripped of its science-fictional jacket, was to reconstruct the game that ended in the diagram position after Black's 13th move. Starting at the end, the last move must have been Q(from a1)xb1 mate. Of Black's other moves, his bishops account for at least four, his knight one, his pawns two, and his queen must have taken at least three to reach a1. Which leaves only two moves for a black piece to have reached a3 where White has made a capture.

Those two moves must have been with the rook from h8 to h3, then over to a3. We now have an essentially complete list of Black's moves: g5, hxg6, Bg7, Nc6, Rh3, Ra3, Ba6-d3-c2 (or b7-e4-c2), Qb8-b2-a1 and finally Qxb1 mate.

Furthermore, hxg6 must be played before the rook can move, and the bishop must vacate c8 before the queen moves. So what happened to the pawn on b7? It must have been captured, but by what? And what happened to White's c-pawn? The answers to all the questions are inextricably intertwined.

Since hxg6 cannot be played before move three, the black rook cannot move to a3 until move six at the earliest. So bxa3 cannot happen before move seven, which leaves no time for a white rook to pop down the b-file and capture on b7.

Attempts to make time for Qxb7 also fail: Black runs out of moves long before the white queen can get there and re-route to g6 to allow the black rook to emerge.

The final possibility is that White's c-pawn captured on b7, then promoted to a piece now on the board. With five moves for promotion, two to come out again, two to get the queen to g6, two king moves, Bb2 and bxa3, White's quota is now complete.

The jigsaw is now complete. All that remains is to find the order to stick the pieces together. There is only one way: 1.c4 g5 2.Qc2 Bg7 3.Qg6 hxg6 4.c5 Rh3 5.c6 Ra3 6.cxb7 Nc6 7.b8(R) Ba6 8.Rb3 Qb8 9.Rh3 Bd3 10.bxa3 Qb2 11.Kd1 Qxa1 12.Bb2 Bc2+ 13.Kc1 Qxb1.

Prizes - a year's subscription to Games & Puzzles magazine - to K R Milner (Malvern), Mike Wright (Humberside) and Tony Freeth (South Ealing).

William Hartston