comb0=: 4 : 0 NB. All size x combinations of i.y
if. (x>:y)+.0=x do. i.(x<:y),x else. (0,.x comb0&.<: y),1+x comb0 y-1 end.
)
combm=: 4 : 0 M.
if. (x>:y)+.0=x do. i.(x<:y),x else. (0,.x combm&.<: y),1+x combm y-1 end.
)
comb=: 4 : 0
k=. i.>:d=.y-x
z=. (d$<i.0 0),<i.1 0
for. i.x do. z=. k ,.&.> ,&.>/\. >:&.> z end.
; z
)
(100) ts '10 comb0 20'
0.457109 3.35573e7
(100) ts '10 combm 20'
7.62e_7 1280
(100) ts '10 comb 20'
0.0212303 4.98343e7