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Retro Strategy (RS)

There are two basic retro conventions concerning conditional moves:
(1) castling is taken to be legal, unless you can prove it's illegal.
(2) en passant is taken to be illegal, unless you can prove it's legal.
When multiple conditional moves interact, there are several alternative protocols which can apply. None of them is "right".
Under the current version of the Codex, the default version is Partial Retro Analysis (PRA) (q.v.)
A synonym for PRA these days is Retro Variants (RV).
When a problem does not yield to PRA, the next thing to be tried is RS. Under this rule, one can play forwards any castling move, and this may reduce the number of possible histories.
A common situation is where two castlings are not both legal: in this case the first to be played will succeed.
A synonym for RS is Post Factum (PF).
There is no way under normal RS to play an uncertain en passant, however a common trick called A Posteriori (AP) allows for an uncertain en passant to be played, as long as future castling play will retrospectively prove the legality of that trick. AP will normally appear in the stipulation.