[zrzashma] Rook en passant

Consider modifying the rules of chess so that when castling, if the rook starts under attack, or moves through a square that is attacked, it can be captured en passant kind of like a pawn.  This mirrors the prohibitions on the king from castling while in check or castling through check.

(Note that, unlike pawn en passant, a rook can also be captured on its original square as well as its passing squares.  Also unlike pawn en passant, any piece can capture the rook.)

Or, since the rook can normally safely fly through attacked squares, the rook can be captured only if its origin square was attacked.  But I don't like this too much: the castling move is inspired by moving inside the castle, not on the battlefield, so the rook move during castling is not its normal "outdoors" move by a chariot but a delicate and vulnerable indoors move.  We could also easily imagine a chess variant with a different starting position in which the piece that swaps with the king during castling is not the rook.

Alternatively, modify the rules so that there are no prohibitions of the king castling while in check or through check.  This would simplify the game.