Merge Sort in Clojure

mergesort.clj

(defn merge-sort
  "Produce a sorted sequence from an input sequence. Sort into ascending order by default, or use optional comparison function."
  ([x] (merge-sort < x))
  ([swo x]
     (letfn [(merge [left right]
               (loop [l left r right result []]
                 (let [l-first (first l) r-first (first r)]
                   (cond (nil? l-first) (concat result r)
                         (nil? r-first) (concat result l)
                         :default (if (swo l-first r-first)
                                    (recur (rest l) r (conj result l-first))
                                    (recur l (rest r) (conj result r-first)))))))
             (sort [coll]
               (let [c (count coll)]
                 (if (< c 2)
                   coll
                   (let [h (/ c 2)]
                     (apply merge (map sort (split-at h coll)))))))]
       (sort x))))