(define (rot13 str)
  (define (rot13-char c)
    (integer->char (+ (char->integer c)
                      (cond ((and (char>=? c #\a) (char<? c #\n))
                             13)
                            ((and (char>=? c #\A) (char<? c #\N))
                             13)
                            ((and (char>=? c #\n) (char<=? c #\z))
                             -13)
                            ((and (char>=? c #\N) (char<=? c #\Z))
                             -13)
                            (else
                             0)))))
  (list->string (map rot13-char (string->list str))))