TCO optimised Clojure implementation of Merge Sort

MerjSawt.clj

(ns merge_sort.core
  (:use [clojure.java.io :only (reader)])
  (:gen-class :main true))

(defn merj [left right]
  (loop [[lhead & ltail :as left] left
         [rhead & rtail :as right] right
         result []]
    (cond (nil? left) (concat result right)
          (nil? right) (concat result left)
          true (if (<= lhead rhead) 
                (recur ltail right (conj result lhead))
                (recur left rtail (conj result rhead))))))

(defn sawt [lst]
  (let [array lst
        middle (bit-shift-right (count array) 1)
        [left right] (split-at middle array)]
    (if (= 1 (count array))
      array
      (merj (sawt left) (sawt right)))))

(defn -main [& args]
  (with-open [rdr (reader "IntegerArray.txt")]
   (println (sawt (map #(Integer/parseInt %) (line-seq rdr))))))