ROT-13 encryption in Clojure

rot13.clj

(defn shift13 [start n]
  (let [s (int start)
        c (int n)]
    (char (if (<= s c (+ s 25))
            (+ s (mod (+ (- c s) 13) 26))
            n))))

(defn rot13 [st]
  (->> (map (fn [x]
              (let [n (int x)]
                (cond
                 (<= 65 n 90) (shift13 \A x)
                 (<= 97 n 122) (shift13 \a x)
                 :else x)))
            (seq st))
       (apply str)))

;; (rot13 "hEllo") => "uRyyb"
;; (rot13 "uRyyb") => "hEllo"
;; (rot13 "The quick brown fox jumps over the lazy dog.") => "Gur dhvpx oebja sbk whzcf bire gur ynml qbt."