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For a change, a problem involving light retroanalysis. A basic chess problem convention is that the diagram position should be legal, in other words one that could be arrived at in the course of a hypothetical game. The solver’s task here is to prove that there is only one square on which the black king can legally stand mated.
The mated king cannot stand on the eighth rank because White’s mating move would have had to be g7xh8=R, but all 8 white pawns are still on the board. A number of squares such as h6 and d5 are ruled out because in each case no white move could have delivered the double check. If the king stood on e4 White's mating move would have had to be either d2-d3, which is illegal because the bishop from c1 could not have moved to f4 earlier, or e2xd3, in which case the bishop from f1 could not have come out earlier to be captured at h5. The king must stand on d4, White having just double-checked by d5xe5 e.p., after Black’s last move e7-e5.
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