Clojure=> (combinations '(a b c) 2)
((a b) (a c) (b c))
Clojure=> (combinations '(a b c d e) 3)
((a b c) (a b d) (a b e) (a c d) (a c e) (a d e) (b c d) (b c e) (b d e) (c d e))