S9 LIB  (combine integer list)   ==>  list
        (combine* integer list)  ==>  list
        
        (load-from-library "combine.scm")

Create k-combinations of the elements of the given list. K (the
size of the combinations) is specified in the integer argument.
COMBINE creates combinations without repetition, and COMBINE*
creates combinations with repetition.

(combine 2 '(a b c))   ==>  ((a b) (a c) (b c))
(combine* 2 '(a b c))  ==>  ((a a) (a b) (a c)
                             (b b) (b c) (c c))
