merge sort in clojure

msort.clj

(defn mrg
  ([L R] (mrg L R []))
  ([[l & left :as L] [r & right :as R] M]
  (if (or (empty? L) (empty? R))
    (concat M L R)
    (if (<= l r)
      (recur left R (conj M l))
      (recur L right (conj M r))))))

(defn msort [arr]
  (if (empty? (rest arr))
    arr
    (let [sides (split-at (/ (count arr) 2) arr)]
      (mrg (msort (sides 0)) (msort (sides 1))))))