(define rot-char-13 (lambda (ch)
  (set! ch (string->rune ch))
  (cond
    (
      (or
        (and (>= ch (string->rune "a")) (<= ch (string->rune "m")))
        (and (>= ch (string->rune "A")) (<= ch (string->rune "M"))))
      (rune->string (+ ch 13)))
    (
      (or
        (and (>= ch (string->rune "n")) (<= ch (string->rune "z")))
        (and (>= ch (string->rune "N")) (<= ch (string->rune "Z"))))
      (rune->string (- ch 13)))
    (else (rune->string ch)))))

(display (list->string (map rot-char-13 (string->list "nowhere->ABJURER!"))))