99 Problems in Lisp (Part VIII)

P26 - Generating the combinations of K distinct objects chosen from N.

(defn append-prefix [prefix lst-elements]
 (mapcat (fn [x] (list (concat prefix (list x)))) lst-elements))

(defn combination [n lst]
 (if (> n (count lst))
   nil
   (let [elem-list (split lst (dec n)) rlist (nthrest lst (dec n))]
     (concat (append-prefix (first elem-list) rlist) (combination n (rest lst))))))

Only two left and I've done the lists part....