Why in the castle subroutine in fairychess.txt include file the line unsetflag #RPOS; is commented?
I suppose it could be uncommented if you wanted to be thorough, but it isn't necessary to prevent illegal castling. Once the King has moved, the space it moves from and the space it moves to both get unflagged, and every space the King moves to gets unflagged. So, no matter what, the King is still unable to castle. As long as the King can't castle, nothing can castle with the King. So, leaving a flag on the Rook's original space doesn't change anything critical to the game.
I suppose it could be uncommented if you wanted to be thorough, but it isn't necessary to prevent illegal castling. Once the King has moved, the space it moves from and the space it moves to both get unflagged, and every space the King moves to gets unflagged. So, no matter what, the King is still unable to castle. As long as the King can't castle, nothing can castle with the King. So, leaving a flag on the Rook's original space doesn't change anything critical to the game.