(define (within? x lower upper)
  (and (<= x upper) (>= x lower)))

(define (wrap-range x lower upper)
  (cond
    ((> x upper) (wrap-range (+ lower (- x upper)) lower upper))
    ((< x lower) (wrap-range (- upper (- lower x)) lower upper))
    (else x))) 

; rotate-char char amount lower upper -> char
; If char is within lower and upper range, increase char by amount, and if
; it goes over the upper range, then wrap around to the lower range.
(define (rotate-char in amount lower upper)
  (let
      ((in (char->integer in))
       (lower (char->integer lower))
       (upper (char->integer upper)))
    (integer->char
     (if (within? in lower upper) 
         (wrap-range (+ in amount) lower upper)
         in))))


(define (rot13 in)
  (cond 
      ((char-lower-case? in) (rotate-char in 13 #\a #\z))
      ((char-upper-case? in) (rotate-char in 13 #\A #\Z))
      (else in)))


; I am not sure I want to think about this for now...
;(define (rotate-char2 in amount lower upper)
;  (let loop ((i in)) 
;    (cond 
;      ((< i lower) (loop (+ i amount)))
;      ((< i upper) i)
;      (else (loop (+ lower (- i upper)))))))