Merge sort in Clojure

merge.clj

(ns merge-sort)

(defn split-at-middle [xs] (split-at (/ (count xs) 2) xs))

(defn merge [X Y]
  (loop [ [xhead & xtail :as x] X, [yhead & ytail :as y] Y, Result []]
    (if (and (not-empty x) (not-empty y))
      (if (<= xhead yhead)
        (recur xtail y (conj Result xhead))
        (recur x ytail (conj Result yhead)))
      (concat Result x y))))

(defn merge-sort [xs]
  (if (or (zero? (count xs)) (= 1 (count xs))) xs
  (let [[xa xb] (split-at-middle xs)]
    (merge (merge-sort xa) (merge-sort xb)))))