(((row-num . 0) ((a . u)) (a b) (a f))
 ((row-num . 1) ((a . d)) (a b) (a f))
 ((row-num . 2) ((b . u)) (a b) (b c))
 ((row-num . 3) ((b . d)) (a b) (b c))
 ((row-num . 4) ((b . u)) (a b) (b e))
 ((row-num . 5) ((b . d)) (a b) (b e))
 ((row-num . 6) ((c . u)) (b c) (c d))
 ((row-num . 7) ((c . d)) (b c) (c d))
 ((row-num . 8) ((c . u)) (b c) (c f))
 ((row-num . 9) ((c . d)) (b c) (c f))
 ((row-num . 10) ((d . u)) (c d) (d e))
 ((row-num . 11) ((d . d)) (c d) (d e))
 ((row-num . 12) ((e . u)) (b e) (d e))
 ((row-num . 13) ((e . d)) (b e) (d e))
 ((row-num . 14) ((e . u)) (b e) (e f))
 ((row-num . 15) ((e . d)) (b e) (e f))
 ((row-num . 16) ((f . u)) (a f) (c f))
 ((row-num . 17) ((f . d)) (a f) (c f))
 ((row-num . 18) ((f . u)) (a f) (e f))
 ((row-num . 19) ((f . d)) (a f) (e f)))