Article 16 – Castling and En-passant capture
(1) Castling convention. Castling is permitted unless it can be proved that it is not permissible.
(2) En-passant convention. An en-passant capture on the first move is permitted only if it can be proved that the last move was the double step of the pawn which is to be captured.
(3) Partial Retrograde Analysis (PRA) convention. 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. In the special case where White’s right to castle excludes Black’s right to castle (and vice versa), an alternative is possible: the party exercising this right first is entitled to castle (Retro Strategy (RS) convention).
(4) Other conventions should be expressly stipulated, for example if in the course of the solution an en-passant capture has to be legalized by subsequent castling (a posteriori (AP) convention).
I find this somewhat unsatisfactory. Let me mention two issues.
First, I find that the different clauses offer different definitions of critical terms, for instance the term "right to castle." We get one convention regarding this in (1). But then, when we get to (3), we are in effect told that if application of (1) leads to a contradiction (say to the claim that both White and Black can castle in a position in which it can be independently shown that both cannot castle), then "an alternative is possible," namely assign the right to castle to the side that moves first. That "alternative" is a different convention governing the expression "right to castle," a convention that is in conflict with a strict application of the convention articulated in (1). In general, it does not seem desirable to lay down conventions which require that one then add clauses that in effect say that, if those conventions lead to trouble, then here are some other conventions one might adopt instead.
Second, I think that this proposal leads to a narrowing of all retroanalytical possibilities. There is a semblance of full generality when (3) enjoins us to consider "All possible combinations of move rights," but it is mere appearance. For the phrase is immediately qualified when we are told to take "into account the castling convention and the en-passant convention." Admittedly, this is rather vague (what does it mean to take into account a convention, let alone a convention whose content is now being modified by a further clause?), but presumably the intention is precisely to stop us from considering "all possible combinations of move rights." For instance, Keym wields (3) in such a way that in Loyd's Texas Siftings 1888 problem (quoted by Keym), we must ignore the possible combination of move rights in which Black cannot castle at all. Partial Retrograde Analysis, so understood, is aptly so-called!
A few years back, I surveyed a number of these issues in the attached essay.
