Memoizing inserts when calculating comb(30,15),
which is 155117520, with 310235039 recursive calls, but 
255 items:

( (15, 0) , 1 )
( (14, 0) , 1 )
( (13, 0) , 1 )
( (12, 0) , 1 )
( (11, 0) , 1 )
( (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 )
( (12, 1) , 12 )
( (13, 1) , 13 )
( (14, 1) , 14 )
( (15, 1) , 15 )
( (16, 1) , 16 )
( (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 )
( (13, 2) , 78 )
( (14, 2) , 91 )
( (15, 2) , 105 )
( (16, 2) , 120 )
( (17, 2) , 136 )
( (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 )
( (14, 3) , 364 )
( (15, 3) , 455 )
( (16, 3) , 560 )
( (17, 3) , 680 )
( (18, 3) , 816 )
( (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 )
( (15, 4) , 1365 )
( (16, 4) , 1820 )
( (17, 4) , 2380 )
( (18, 4) , 3060 )
( (19, 4) , 3876 )
( (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 )
( (16, 5) , 4368 )
( (17, 5) , 6188 )
( (18, 5) , 8568 )
( (19, 5) , 11628 )
( (20, 5) , 15504 )
( (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 )
( (17, 6) , 12376 )
( (18, 6) , 18564 )
( (19, 6) , 27132 )
( (20, 6) , 38760 )
( (21, 6) , 54264 )
( (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 )
( (18, 7) , 31824 )
( (19, 7) , 50388 )
( (20, 7) , 77520 )
( (21, 7) , 116280 )
( (22, 7) , 170544 )
( (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 )
( (19, 8) , 75582 )
( (20, 8) , 125970 )
( (21, 8) , 203490 )
( (22, 8) , 319770 )
( (23, 8) , 490314 )
( (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 )
( (20, 9) , 167960 )
( (21, 9) , 293930 )
( (22, 9) , 497420 )
( (23, 9) , 817190 )
( (24, 9) , 1307504 )
( (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 )
( (21, 10) , 352716 )
( (22, 10) , 646646 )
( (23, 10) , 1144066 )
( (24, 10) , 1961256 )
( (25, 10) , 3268760 )
( (11, 11) , 1 )
( (12, 11) , 12 )
( (13, 11) , 78 )
( (14, 11) , 364 )
( (15, 11) , 1365 )
( (16, 11) , 4368 )
( (17, 11) , 12376 )
( (18, 11) , 31824 )
( (19, 11) , 75582 )
( (20, 11) , 167960 )
( (21, 11) , 352716 )
( (22, 11) , 705432 )
( (23, 11) , 1352078 )
( (24, 11) , 2496144 )
( (25, 11) , 4457400 )
( (26, 11) , 7726160 )
( (12, 12) , 1 )
( (13, 12) , 13 )
( (14, 12) , 91 )
( (15, 12) , 455 )
( (16, 12) , 1820 )
( (17, 12) , 6188 )
( (18, 12) , 18564 )
( (19, 12) , 50388 )
( (20, 12) , 125970 )
( (21, 12) , 293930 )
( (22, 12) , 646646 )
( (23, 12) , 1352078 )
( (24, 12) , 2704156 )
( (25, 12) , 5200300 )
( (26, 12) , 9657700 )
( (27, 12) , 17383860 )
( (13, 13) , 1 )
( (14, 13) , 14 )
( (15, 13) , 105 )
( (16, 13) , 560 )
( (17, 13) , 2380 )
( (18, 13) , 8568 )
( (19, 13) , 27132 )
( (20, 13) , 77520 )
( (21, 13) , 203490 )
( (22, 13) , 497420 )
( (23, 13) , 1144066 )
( (24, 13) , 2496144 )
( (25, 13) , 5200300 )
( (26, 13) , 10400600 )
( (27, 13) , 20058300 )
( (28, 13) , 37442160 )
( (14, 14) , 1 )
( (15, 14) , 15 )
( (16, 14) , 120 )
( (17, 14) , 680 )
( (18, 14) , 3060 )
( (19, 14) , 11628 )
( (20, 14) , 38760 )
( (21, 14) , 116280 )
( (22, 14) , 319770 )
( (23, 14) , 817190 )
( (24, 14) , 1961256 )
( (25, 14) , 4457400 )
( (26, 14) , 9657700 )
( (27, 14) , 20058300 )
( (28, 14) , 40116600 )
( (29, 14) , 77558760 )
( (15, 15) , 1 )
( (16, 15) , 16 )
( (17, 15) , 136 )
( (18, 15) , 816 )
( (19, 15) , 3876 )
( (20, 15) , 15504 )
( (21, 15) , 54264 )
( (22, 15) , 170544 )
( (23, 15) , 490314 )
( (24, 15) , 1307504 )
( (25, 15) , 3268760 )
( (26, 15) , 7726160 )
( (27, 15) , 17383860 )
( (28, 15) , 37442160 )
( (29, 15) , 77558760 )
( (30, 15) , 155117520 )