This exercise in retro-analysis (composed by N Petrovic) is a variation on a well-known theme. The question is: what were the last six moves (three White and three Black)?

Black is in check, but White's bishop cannot have moved to a1 without Black having already been in check. It cannot have been a discovered check for the same reason. If you've seen this sort of thing before, you'll quickly recognise that there's only one possibility: White has just made an en passant capture of a pawn on e5. Black's previous move was ...e7- e5, to which White replied either fxe6 or dxe6.

But how did the bishop deliver check before ...e7-e5? The only possibility is that the white pawn was on d4 and discovered check by moving to d5. Now comes the really clever bit. With white pawn on d4, black pawn on e7, what was Black's last move? Any move of the king seems to be from an impossible double check. There is only one way out of the dilemma. From the diagram, move the black king on e6, add black pawns on f7 and e7, white ones on e5 and d4. Play continues 1...f5 2.exf6+ Kxf6 3.d5+ e5 4.dxe6+ and we're home! A splendid double-retro-en passant!