Editing Retrograde analysis (section)

===Partial retrograde analysis (PRA)===
{{Chess diagram
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| '''W. Langstaff''', ''Chess Amateur'' 1922

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|  |  |  |rl|  |kl|pd|pl
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| '''Mate in two.''' <br>This problem uses ''partial retrograde analysis'' method.
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Some problems use a method called "partial retrograde analysis" (PRA). In these, the history of a position cannot be determined with certainty, but each of the alternative histories demands a different solution. In Article 16 of the ''Codex for Chess Composition'', the PRA convention is formally defined as follows:

{{blockquote|Where the rights to castle and/or to capture en-passant are mutually dependent, the solution consists of several mutually exclusive parts. All possible combinations of move rights, taking into account the castling convention and the en-passant convention, form these mutually exclusive parts.}}

The problem to the left by W. Langstaff (from ''Chess Amateur'' 1922) is a relatively simple example; it is a mate in two. It is impossible to determine what move Black played last, but it is clear that he must have either moved the king or rook, or else played g7–g5 (g6–g5 is impossible, since the pawn would have been giving check). Therefore, either Black cannot castle, or White can capture on g6 ''en passant''. It is impossible to determine exactly what Black's last move actually was, so the solution has two lines:

{{block indent|
1.Ke6 and 2.Rd8# (if Black moved the king or rook)<br>
1.hxg6 e.p. (threat: 2.Rd8#) 1...0-0 2.h7# (if Black played g7–g5)
}}
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