Single Comment

When you're implementing the rules of a game, you have to pay closer
attention to the consequences of the rules than you would have to just to
learn the game. This page omits an important detail I had to discover on
my own. En passant is possible only for a Pawn on the second board. When a
Pawn makes a double move, it moves to the second board. If it had moved
only one space, it would have still moved to the second board, and only a
Pawn on that board would have been able to capture it. Since en passant is
supposed to allow a Pawn to capture an enemy Pawn it would have been able
to capture if it had moved only one space, it follows that en passant is
for the Pawn waiting on the second board, not for any Pawn on the first
board.

Furthermore, I have deduced that a Pawn can be properly situated for
making an en passant capture only if it has never made a double move. To
be properly situated, a Pawn must be on a player's fifth rank. To get to
the fifth rank, a Pawn may make three single space moves or a double space
move and a single space move. With three single space moves, the Pawn will
be on the second board. But if it makes a double move and a single move,
its two moves will return it to the first board, and it will be unable to
capture anything by en passant.