Square rfrom = to; // 'King captures the rook' notation
Square kto = relative_square(us, rfrom > kfrom ? SQ_G1 : SQ_C1);
Square rto = relative_square(us, rfrom > kfrom ? SQ_F1 : SQ_D1);
Square rfrom = to; // 'King captures the rook' notation
Square kto = relative_square(us, rfrom > kfrom ? SQ_G1 : SQ_C1);
Square rto = relative_square(us, rfrom > kfrom ? SQ_F1 : SQ_D1);