Merge sort implementation in Clojure. Merge step is recursive itself!

mergesort.clj

(defn mergestep [l r op]
  (cond (empty? l) r
        (empty? r) l
        :else
           (let [fl (first l)
                 fr (first r)]
             (if (op fl fr)
               (cons fl (mergestep (rest l) r op))
               (cons fr (mergestep l (rest r) op))))))
 
 
(defn mergesort 
  ([data]
     (mergesort data <))
  ([data op]
    (if (< (count data) 2)
      data
      (let [halfsize (/ (count data) 2)
            split (split-at halfsize data)
            left (first split)
            right (second split)
            result (mergestep (mergesort left op) (mergesort right op) op)]
        result))))

(mergesort '(7 8 12  2 3 45)) ; (2 3 7 8 12 45)
(mergesort '(7 8 12  2 3 45) >) ; (45 12 8 7 3 2)