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"Curses! The fiend has foiled us again." Dr Watson made no secret of his frustration. "Professor Moriarty has escaped."

"It is my fault," said his companion. "Had I not taken precious minutes to explain to you how I solved the curious case of the Martian omnibus, we might have arrived in time."

"Yet now all we have is this position and a report that Moriarty missed an instant checkmate six moves ago." Watson moved to put away the pieces, but was stopped in his tracks by an urgently outstretched hand.

"Wait!" shouted the great detective. For I can tell you the exact move he missed."

"That's impossible, Holmes," said Watson, obtuse as ever.

"Count the captures," urged Holmes. "A black pawn has made four in trekking from e7 to a3, which accounts for all the missing white men. And a white pawn, on its march from c2 to h7, has eliminated all the black absentees, save the bishop which perished on c8."

"That tells us nothing of a mate," interrupted the doctor.

"Patience," admonished the detective. "Since Black is in check, White made the last move, which perforce was with the rook from f2 to f1. But what was Black's preceding move?"

"Well, his rooks, bishop and knight clearly cannot have moved. And his king cannot have moved from f1 without having been in a check that could never have been delivered. So it must have been a pawn capture from b4 to a3."

"Where it took a white pawn," added Holmes. "Indeed we may deduce with certainty that it captured pawns on a3 and b4, and knights on c5 and d6. Equally the h7-pawn captured pawns on f5, g6 and h7."

"Which gets us no nearer a mate," said Watson irritably.

Holmes ignored him: "So taking the present position as move zero, we have moves minus-one as Rf2-f1+ for White and b4xPa3 for Black. We must now proceed backwards with care, lest we find in three moves time that Black had no possible last move."

"Good God! do you mean..."

"Yes, Watson, a retro- self-stalemate! Which will happen unless the previous moves were as follows: minus-2, g6xPh7 and c5xPb4; minus-3, f5xPg6 and d6xNc5; minus-4, e4xPf5 and e7xNd6. White's minus-fifth must have been d3xNe4, to permit Black the minus-fifth move of Ng5-e4! Knowing that Black missed a mate on move minus-6, we can now complete the task with c2xQd3 and Qc3-d3+. With the position reconstructed, Holmes pushed the pawn from c7 to c6. "Checkmate, Moriarty."