Alice chess is played on 2 boards. In the initial position, all pieces
are on board A:
After moving a piece (begin- and end-square are on the same board),
the piece is moved to the other board. The square it is moved to must
be empty. So a game could for instance start with 1. d2-d4 (->B) g7-g5
(->B) 2. Bc1-h6 (->B) going "through" the pawn on the other board!
Every move must be legal on the board it is played on. So a king can't
walk into check given on the board the move is made on, even though
the check is only virtual as he is transferred to the other board and
can't be captured.