+ case DELTA_N:
+ case DELTA_S:
+ // Pawn push. The destination square must be empty.
+ if (!square_is_empty(to))
+ return false;
+ break;
+
+ case DELTA_NN:
+ // Double white pawn push. The destination square must be on the fourth
+ // rank, and both the destination square and the square between the
+ // source and destination squares must be empty.
+ if ( square_rank(to) != RANK_4
+ || !square_is_empty(to)
+ || !square_is_empty(from + DELTA_N))
+ return false;
+ break;
+
+ case DELTA_SS:
+ // Double black pawn push. The destination square must be on the fifth
+ // rank, and both the destination square and the square between the
+ // source and destination squares must be empty.
+ if ( square_rank(to) != RANK_5
+ || !square_is_empty(to)
+ || !square_is_empty(from + DELTA_S))
+ return false;
+ break;