🕸📝Fergus Duniho wrote on Mon, Mar 25, 2013 04:00 PM UTC:
Note that the "legal if strictly legal on the board it is played" interpretation is flawed anyway, because it would allow you to legally put your own King in check: white Ke1, Pe2, black Qe8 (all on same board) would have e2-e4 legal, because before you transfer e4 to e4*, your King is not yet in check.
I was never even considering that interpretation. The two interpretations I'm considering are (1a) A move is not legal unless it is strictly legal on its board, including not leaving the King in check even temporarily and (1b) A move is not legal when, considering the board it moved on as the only board in the game, that move would be considered illegal. Both of these are only necessary conditions on whether a move is legal. With either interpretation of the rule, you could not put your King in check, because there is already another rule against that.
The second interpretation, which is the one you agree with, makes more sense of the clause "on the board where it is played." If we consider this clause to be limiting the domain we're going to look at to determine the legality of the move, this rule is not redundant. But on the strict legality interpretation, the rule amounts to nothing more than "a move is not legal unless it is legal." But that interpretation actually leaves things ambiguous. It is trying to define legality in Alice Chess in terms of legality in Alice Chess, leaving open the question of what is really legal in Alice Chess. 1b defines legality non-recursively by defining legality in Alice Chess in terms of legality in Chess, as played on a single board. So, on 1b, leaving the King in check makes a move illegal only if the King would be left in check on that board before the piece moved to the other board. Whether the King is left in check on the other board does not affect the legality of the move. So, a piece should be able to block a check by moving from the board the King is not on.
I was never even considering that interpretation. The two interpretations I'm considering are (1a) A move is not legal unless it is strictly legal on its board, including not leaving the King in check even temporarily and (1b) A move is not legal when, considering the board it moved on as the only board in the game, that move would be considered illegal. Both of these are only necessary conditions on whether a move is legal. With either interpretation of the rule, you could not put your King in check, because there is already another rule against that.
The second interpretation, which is the one you agree with, makes more sense of the clause "on the board where it is played." If we consider this clause to be limiting the domain we're going to look at to determine the legality of the move, this rule is not redundant. But on the strict legality interpretation, the rule amounts to nothing more than "a move is not legal unless it is legal." But that interpretation actually leaves things ambiguous. It is trying to define legality in Alice Chess in terms of legality in Alice Chess, leaving open the question of what is really legal in Alice Chess. 1b defines legality non-recursively by defining legality in Alice Chess in terms of legality in Chess, as played on a single board. So, on 1b, leaving the King in check makes a move illegal only if the King would be left in check on that board before the piece moved to the other board. Whether the King is left in check on the other board does not affect the legality of the move. So, a piece should be able to block a check by moving from the board the King is not on.