S9 LIB  (choose integer1 integer2)  ==>  integer

Return the binomial coefficient ("n choose k") of INTEGER1 (n)
and INTEGER2 (k), i.e. the number of unique k-tuples that can be
choosen without repetition from a set of n elements. INTEGER2 (k)
must be positive.

(choose 23 1)  ==>  23
(choose 23 2)  ==>  253
(choose 23 3)  ==>  1771
