H. G. Muller wrote on Mon, Mar 25, 2013 12:43 PM UTC:
If that is true, the description of the rules as given here ("the move must be legal on the board where it starts") is wrong. Funny enough my Alice-Chess engine seems to accept them, though, as it is a King-capture engine. So it first makes the full move (including the board transfer), and then checks if the reply can capture a King. The only thing special I programmed is that it also triggers on capturing to an empty square if that square has a King on the other board that just moved. (I.e. it is as if the King on moving leaves a 'ghost' on the square through which it passed, and can be captured e.p. there.) This would not flag interposing as illegal, however.
It seems a proper formulation of the rules would be: "the move must be pseudo-legal (i.e. legal when ignoring check) on the board where it is made, but when a King moves it must not pass through check on that board."
If that is true, the description of the rules as given here ("the move must be legal on the board where it starts") is wrong. Funny enough my Alice-Chess engine seems to accept them, though, as it is a King-capture engine. So it first makes the full move (including the board transfer), and then checks if the reply can capture a King. The only thing special I programmed is that it also triggers on capturing to an empty square if that square has a King on the other board that just moved. (I.e. it is as if the King on moving leaves a 'ghost' on the square through which it passed, and can be captured e.p. there.) This would not flag interposing as illegal, however. It seems a proper formulation of the rules would be: "the move must be pseudo-legal (i.e. legal when ignoring check) on the board where it is made, but when a King moves it must not pass through check on that board."