H. G. Muller wrote on Mon, May 4, 2020 09:33 PM UTC:
So, by the definition you gave here, it has the pseudo-legal move to capture exactly as a Rook does.
That is correct, and intended. Rules about passing through check are on par with rules about moving into check. It is often easiest to interpret passing through check as exposing the royal to (generalized) e.p. capture. The latter kicks in when a pseudo-legal move is defined as 'composite' rather than 'monolithic'. (I make these terms up as I go...) There then is an extra rule that in the next turn the opponent has the right to partly retract an immediately preceding composite move before making his own move, provided the latter captures the moving piece (perhaps only with a sub-set of the piece types). In orthodox Chess the Pawn double-push and castling are composit moves, and the opponent can make the doubly-pushed Pawn go back to the square it passed through, and capture it there in the normal way (but only with a Pawn). After castling, he can 'uncastle' to the point where the King has made the first step, and then capture the King there with any piece. With as a result that the castling was illegal, as you are not allowed to expose your King to capture.
Passing through check, like for the Caissa Brittania Queen, could work the same. The pseudo-legal move does pass through check, but it is composit. If an actual move would pass through check, the opponent would undo it to the point where the Queen was in check, and capture it there. (Again any piece type could do this.) Thus demonstrating that the move that passed through check exposed the Queen to capture, and thus was illegal. The necessary condition, however, is that the opponent should get to move again. If there is no after-move after capturing a royal, Q x Q could pass through check safely. The possibility to recapture e.p. is worth just as little as the possibility to recapture in the ordinary way, when you lost your royal.
If in Fusion Chess the right to split up depends on being attacked, and there are pieces that are not allowed to pass through check, it is up to the designer to decide if the splitting rights are derived from pseudo-legal attack, or from legal attack only.
I had a similar dilemma in Mighty Lion Chess. I wanted to have the Chu-Shogi rule that Lions cannot capture protected Lions in a Chess-like game (that has checking rules). For simplicity I decided that pseudo-legal protection would be enough. Which is equivalent to saying that there is no after-move when a Lion that just captured a Lion is recaptured. So you don't have to worry about exposing your King in check when recapturing that Lion, it is immediately decisive, just like when you capture a King. This rule makes the Lion effectively a second (absolute) royal for one turn after Ln x Ln.
To comment on Greg's point: Fusion Chess (old rules) would be double complicated, because the legality involves not only exposure of the royal, but also has to take account of whether it passed through check. The latter is already a pain in itself, because you would also have to test all the square on the path of the royal after it moved, not just where it is now. Probably testing this afterwards, as my engines do for normal King exposure, is no longer efficient, since many moves of the royal might all pass through the same check. Same for the split: all moves of the parts would become illegal together when you detect an attack. I guess you ccould exploit the null move here very well: if every node in the full-width search starts searching a null move, the move generation for the null-move reply would detect all pseudo-legal attacks on fused pieces. And even when the null move drops you directly into QS, you would search these moves, since these are all captures. In that QS any captures by royals would have to be vetted for not passing through check, and they could return the score for 'illegal move' when they do pass through or move into check. The null-move could then discard those pseudo-legal attacks. Only problem is that when you get a beta cutoff, you know that the null move has failed low, and thatthe parent node will now have to do a normal search. For which it has to know the legality of all the splits. So you would be forced to search all captures of fused pieces in the null-move reply, even after beta cutoff. (Or at least judge their legality, which probably requires making them.) That is, as soon as you have found one legal capture on a fusion piece, you can ignore all other pseudo-legal captures on it if the score is already above beta. In the end you would know which fusion pieces are under legal attack, pass that info back to the parent, who would then use it during its move generation. As I understand that you cannot capture with a split, there is no splitting in QS, and it doesn't matter that you don't search null moves there.
That is correct, and intended. Rules about passing through check are on par with rules about moving into check. It is often easiest to interpret passing through check as exposing the royal to (generalized) e.p. capture. The latter kicks in when a pseudo-legal move is defined as 'composite' rather than 'monolithic'. (I make these terms up as I go...) There then is an extra rule that in the next turn the opponent has the right to partly retract an immediately preceding composite move before making his own move, provided the latter captures the moving piece (perhaps only with a sub-set of the piece types). In orthodox Chess the Pawn double-push and castling are composit moves, and the opponent can make the doubly-pushed Pawn go back to the square it passed through, and capture it there in the normal way (but only with a Pawn). After castling, he can 'uncastle' to the point where the King has made the first step, and then capture the King there with any piece. With as a result that the castling was illegal, as you are not allowed to expose your King to capture.
Passing through check, like for the Caissa Brittania Queen, could work the same. The pseudo-legal move does pass through check, but it is composit. If an actual move would pass through check, the opponent would undo it to the point where the Queen was in check, and capture it there. (Again any piece type could do this.) Thus demonstrating that the move that passed through check exposed the Queen to capture, and thus was illegal. The necessary condition, however, is that the opponent should get to move again. If there is no after-move after capturing a royal, Q x Q could pass through check safely. The possibility to recapture e.p. is worth just as little as the possibility to recapture in the ordinary way, when you lost your royal.
If in Fusion Chess the right to split up depends on being attacked, and there are pieces that are not allowed to pass through check, it is up to the designer to decide if the splitting rights are derived from pseudo-legal attack, or from legal attack only.
I had a similar dilemma in Mighty Lion Chess. I wanted to have the Chu-Shogi rule that Lions cannot capture protected Lions in a Chess-like game (that has checking rules). For simplicity I decided that pseudo-legal protection would be enough. Which is equivalent to saying that there is no after-move when a Lion that just captured a Lion is recaptured. So you don't have to worry about exposing your King in check when recapturing that Lion, it is immediately decisive, just like when you capture a King. This rule makes the Lion effectively a second (absolute) royal for one turn after Ln x Ln.
To comment on Greg's point: Fusion Chess (old rules) would be double complicated, because the legality involves not only exposure of the royal, but also has to take account of whether it passed through check. The latter is already a pain in itself, because you would also have to test all the square on the path of the royal after it moved, not just where it is now. Probably testing this afterwards, as my engines do for normal King exposure, is no longer efficient, since many moves of the royal might all pass through the same check. Same for the split: all moves of the parts would become illegal together when you detect an attack. I guess you ccould exploit the null move here very well: if every node in the full-width search starts searching a null move, the move generation for the null-move reply would detect all pseudo-legal attacks on fused pieces. And even when the null move drops you directly into QS, you would search these moves, since these are all captures. In that QS any captures by royals would have to be vetted for not passing through check, and they could return the score for 'illegal move' when they do pass through or move into check. The null-move could then discard those pseudo-legal attacks. Only problem is that when you get a beta cutoff, you know that the null move has failed low, and thatthe parent node will now have to do a normal search. For which it has to know the legality of all the splits. So you would be forced to search all captures of fused pieces in the null-move reply, even after beta cutoff. (Or at least judge their legality, which probably requires making them.) That is, as soon as you have found one legal capture on a fusion piece, you can ignore all other pseudo-legal captures on it if the score is already above beta. In the end you would know which fusion pieces are under legal attack, pass that info back to the parent, who would then use it during its move generation. As I understand that you cannot capture with a split, there is no splitting in QS, and it doesn't matter that you don't search null moves there.