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I just fixed a bug in this ZRF and uploaded a new version. The bug showed up in a game that Antoine Fourriere sent me. At one point in the game, Black had checkmated White, but the game continued anyway. White's next move was to block the Black Queen's check by moving a Pawn. But this move should have been illegal, because the Pawn was supposed to move to the other board after temporarily occupying the space it would need to occupy to block the check. Instead, Zillions let it stop there without moving to the other board. To fix this bug, I gave all pieces but the King an invisible? attribute. Except for King moves and castling, any piece that moved would have this attribute set to true on completing the first part of its move, and it would have it set back to false on moving to the other board. I then allowed Rooks, Bishops, and Queens to treat spaces with invisible pieces the same as empty spaces. This removed a piece's ability to block checks on the board it is moving from, forcing such moves to be illegal from the start. After this, Antoine's game concluded in checkmate at the point it should have.