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<P>In games likes <A HREF="/play/fischer_random_chess.html">Fischer Random Chess</A>, which allows a King to castle by moving one space or no spaces, I handle castling as a Rook move, because that makes it unambiguous. For en passant moves, I record that the piece at a certain space can be captured by en passant, and then I test whether any neighboring Pawns can take it by en passant. This is what I do in <A HREF="/play/gross_chess.html">Gross Chess</A>, for example. I have had to store multiple en passant locations only for <A HREF="/play/marseillais_chess.html">Marseillais Chess</A>, which allows a player to make two double moves with Pawns in the same turn and allows the other player to capture both by en passant.</P>