Combinatorics
comb=: [:;(,.&.><@:>:@;\.)^:(i.@>:@-~`[`(1:<@i.@{.~<:@-,2:))m comb n generates all possible choices of m elements from i.n
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 )
An equivalent verb from the for. page of the J dictionary.