Memoizing inserts when calculating comb(20,10),
which is 184756, with 369511 recursive calls, but 
120 items:

( (10, 0) , 1 )
( (9, 0) , 1 )
( (8, 0) , 1 )
( (7, 0) , 1 )
( (6, 0) , 1 )
( (5, 0) , 1 )
( (4, 0) , 1 )
( (3, 0) , 1 )
( (2, 0) , 1 )
( (1, 0) , 1 )
( (1, 1) , 1 )
( (2, 1) , 2 )
( (3, 1) , 3 )
( (4, 1) , 4 )
( (5, 1) , 5 )
( (6, 1) , 6 )
( (7, 1) , 7 )
( (8, 1) , 8 )
( (9, 1) , 9 )
( (10, 1) , 10 )
( (11, 1) , 11 )
( (2, 2) , 1 )
( (3, 2) , 3 )
( (4, 2) , 6 )
( (5, 2) , 10 )
( (6, 2) , 15 )
( (7, 2) , 21 )
( (8, 2) , 28 )
( (9, 2) , 36 )
( (10, 2) , 45 )
( (11, 2) , 55 )
( (12, 2) , 66 )
( (3, 3) , 1 )
( (4, 3) , 4 )
( (5, 3) , 10 )
( (6, 3) , 20 )
( (7, 3) , 35 )
( (8, 3) , 56 )
( (9, 3) , 84 )
( (10, 3) , 120 )
( (11, 3) , 165 )
( (12, 3) , 220 )
( (13, 3) , 286 )
( (4, 4) , 1 )
( (5, 4) , 5 )
( (6, 4) , 15 )
( (7, 4) , 35 )
( (8, 4) , 70 )
( (9, 4) , 126 )
( (10, 4) , 210 )
( (11, 4) , 330 )
( (12, 4) , 495 )
( (13, 4) , 715 )
( (14, 4) , 1001 )
( (5, 5) , 1 )
( (6, 5) , 6 )
( (7, 5) , 21 )
( (8, 5) , 56 )
( (9, 5) , 126 )
( (10, 5) , 252 )
( (11, 5) , 462 )
( (12, 5) , 792 )
( (13, 5) , 1287 )
( (14, 5) , 2002 )
( (15, 5) , 3003 )
( (6, 6) , 1 )
( (7, 6) , 7 )
( (8, 6) , 28 )
( (9, 6) , 84 )
( (10, 6) , 210 )
( (11, 6) , 462 )
( (12, 6) , 924 )
( (13, 6) , 1716 )
( (14, 6) , 3003 )
( (15, 6) , 5005 )
( (16, 6) , 8008 )
( (7, 7) , 1 )
( (8, 7) , 8 )
( (9, 7) , 36 )
( (10, 7) , 120 )
( (11, 7) , 330 )
( (12, 7) , 792 )
( (13, 7) , 1716 )
( (14, 7) , 3432 )
( (15, 7) , 6435 )
( (16, 7) , 11440 )
( (17, 7) , 19448 )
( (8, 8) , 1 )
( (9, 8) , 9 )
( (10, 8) , 45 )
( (11, 8) , 165 )
( (12, 8) , 495 )
( (13, 8) , 1287 )
( (14, 8) , 3003 )
( (15, 8) , 6435 )
( (16, 8) , 12870 )
( (17, 8) , 24310 )
( (18, 8) , 43758 )
( (9, 9) , 1 )
( (10, 9) , 10 )
( (11, 9) , 55 )
( (12, 9) , 220 )
( (13, 9) , 715 )
( (14, 9) , 2002 )
( (15, 9) , 5005 )
( (16, 9) , 11440 )
( (17, 9) , 24310 )
( (18, 9) , 48620 )
( (19, 9) , 92378 )
( (10, 10) , 1 )
( (11, 10) , 11 )
( (12, 10) , 66 )
( (13, 10) , 286 )
( (14, 10) , 1001 )
( (15, 10) , 3003 )
( (16, 10) , 8008 )
( (17, 10) , 19448 )
( (18, 10) , 43758 )
( (19, 10) , 92378 )
( (20, 10) , 184756 )