; This is a function that takes a string and returns it rot13-encoded
; example:
; (rot13 "grkg gb qrpbqr")

(define rot13 (lambda (text)

    (define char (lambda (c)
      (string (integer->char
            (cond ((and (char>=? c #\a) (char<=? c #\z))
                   (+ (remainder(+ (- (char->integer c) (char->integer #\a))
                             13) 26 ) (char->integer #\a)))
              ((and (char>=? c #\A) (char<=? c #\Z))
               (+ (remainder(+ (- (char->integer c) (char->integer #\A))
                        13) 26 ) (char->integer #\A)))
                      (else (char->integer c))
            )
          ))
    ))

    (if (> (string-length text) 1)
        (string-append (char (string-ref text 0))
                       (rot13 (substring text 1 (string-length text))))
            (char (string-ref text 0))
    )
))