Merge sort in Clojure

mergesort.clj

; Example usage: (mergesort [5 1 8 16 25 11 4])
(ns mergesort
  (:use clojure.contrib.math))

(defn merge-two
  "Merge two (already sorted) vectors"
  [vec1 vec2]
  (let [v1f (first vec1) v2f (first vec2)]
    (cond
      (empty? vec1) vec2
      (empty? vec2) vec1
      (< v1f v2f)
        (cons v1f (merge-two (rest vec1) vec2))
      :else
        (cons v2f (merge-two (rest vec2) vec1)))))

(defn mergesort
  "Sort a vector of numbers using the merge sort algorithm"
  [vec]
  (let [vecsize (count vec) seam (ceil (/ vecsize 2))]
    (cond
      (= vecsize 1) (vector (get vec 0))
      (= vecsize 2)
        (do
          (if (< (get vec 0) (get vec 1))
            (vector (get vec 0) (get vec 1))
            (vector (get vec 1) (get vec 0))))
      :else
          (merge-two
            (mergesort (subvec vec 0 seam))
            (mergesort (subvec vec seam))))))