Combinations generator in Clojure

combinations.clj

(defn combinations [lst k]
  (letfn [(combinator [x xs]
            (if (= (count x) k)
              [x]
              (when (not (empty? xs))
                (concat (combinator (concat x [(first xs)]) (rest xs))
                        (combinator x (rest xs))))))]
    (combinator nil lst)))

;; user> (combinations (range 1 6) 2)
;; ((1 2) (1 3) (1 4) (1 5) (2 3) (2 4) (2 5) (3 4) (3 5) (4 5))