(define (rot13-char c)
  (cond
   ((char-alphabetic? c)
    (let ((upper? (char-upper-case? c))
           (c (char-downcase c)))
      ((if upper?
           char-upcase
           values)
       (integer->char (+ (char->integer #\a)
                         (remainder (+ 13 (- (char->integer c)
                                             (char->integer #\a))) 26))))))
   (else
    c)
   ))