Editing Retrograde analysis (section)

===The a posteriori (AP) convention===
{{Chess diagram
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| '''N. Petrović''', ''WCCT'' 1967
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| '''White to play and win.''' <br>This problem uses the ''a posteriori'' convention.
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This is perhaps the most controversial of the retrograde analysis conventions; if it is employed, the problem is usually marked as "AP".

Sometimes it is possible to prove that ''if'' castling is possible, then the previous move must have been a double step of a pawn, making an ''en passant'' capture legal. In this case, the ''en passant'' capture is made, then its legality is proved ''a posteriori''; this is accomplished by castling. In some such problems, Black's defence consists of trying to prevent White from castling, rendering the initial ''en passant'' capture illegal. [[Nenad Petrović (chess composer)|Nenad Petrović]] composed several problems in this vein; the example given on the left was discussed extensively in Tim Krabbé's book ''Chess Curiosities''.

Black has made 6 captures; to account for his pawn formation, all 6 captures must have been made with pawns; the capturing pawns must have started on b7, c7, d7 and e7. White has made 4 captures; again all of these captures must have been made with pawns. 3 of these captures must have been made with the pawn on e6, which started on b2. Which of the pawns on f5 and f7 started on g2? Only one of these pawns made a capture; Black's original f-pawn has not made a capture and is still on the f-file, so the pawn on f7 cannot be the original f-pawn; it must have started on g2, and the pawn on f5 must have started on f2. The capture must have been made on f7, after Black played f7–f6 but before Black moved the g-pawn. The sequence was as follows: White's pawns advanced to f5 and g6; Black made a capture exf4; at some point Black played f7–f6; White made a capture gxf7; Black played g7–g5 (or g7–g6 and g6–g5).

In order to solve this problem, it must be considered what White's last move was. If the king or rook moved, then White cannot castle. It has been established that the f5 pawn started on f2, so the only way White could have moved a pawn is if the last move was gxf7, to which Black has immediately replied ...g7–g5. If this is in fact the case, then White can play 1.fxg6 (an ''en passant'' capture). It has thus been proved that ''if'' White can castle, then 1.fxg6 ep is legal.

The solution as originally given was 1.fxg6 ep (intending to prove its legality ''a posteriori'' by castling) 1...Bc5 (preventing castling and threatening ...Bf2+, which would force a king move and delegitimize the ''en passant'' capture) 2.e3 fxe3 3.0-0 (sacrificing a rook in order to legitimize the ''en passant'' capture; if 3.d4 Bb4+ forces a king move and prevents castling) ...e2+ 4.Kg2 exf1=Q+ 5.Kxf1 and White has a won position.

This composition was highly controversial when first published, due in part to the "non-chess" motivations behind the moves 1...Bc5, 2.e3 and 3.0-0, and provoked heated debate in chess problemist circles. Amid the controversy, it was overlooked that the win is not clear in the final position, and in fact Black could have won with 3...exd2+! (instead of 3...e2+) 4.Kg2 e3.