(((row-num . 0) ((a . u)) (a b) (a d))
 ((row-num . 1) ((a . d)) (a b) (a d))
 ((row-num . 2) ((b . u)) (a b) (b c))
 ((row-num . 3) ((b . d)) (a b) (b c))
 ((row-num . 4) ((c . u)) (b c) (c d))
 ((row-num . 5) ((c . d)) (b c) (c d))
 ((row-num . 6) ((d . u)) (c d) (a d))
 ((row-num . 7) ((d . d)) (c d) (a d)))