Playing with CCL, the Mule Code Conversion Language, I decided to write a simple ROT13 algorithm.  This turned out to be tougher than expected since CCL has no concept of the logical and and or operators; what is && and || in C derived languages.

(define-ccl-program my-ccl-rot13
 '(1
   ((loop
     (read r0)
     ;; The convoluted logic of rot13 in CCL.
     (if (r0 >= 110) ; n
       (if (r0 <= 122) ; z
           ((r0 -= 13))) ;; If we are between n-z
         (if (r0 >= 97) ; a
             (if (r0 <= 109) ; m
                 ((r0 += 13))) ;; If we are between a-m
           (if (r0 >= 78) ; N
               (if (r0 <= 90) ; Z
                   ((r0 -= 13))) ;; If we are between N-Z
             (if (r0 >= 65) ; A
                 (if (r0 <= 77); ; M
                     ((r0 += 13))))))) ;; If we are between A-M
     (write r0)
     (repeat)))))

One method to tackle this, as demonstrated above, is to use the if-else construct and carefully order the comparisons.

Now, in order to do something useful with this, we define the function my-rot13-word-at-point.

(defun my-rot13-word-at-point ()
  (interactive)
  (let ((word (word-at-point)))
    (if (stringp word)
	(message (ccl-execute-on-string 'my-ccl-rot13 [0 0 0 0 0 0 0 0 nil]
					word)))))

And now we can do M-x my-rot13-word-at-point and we will see the word (de)cyphered in the echo area.

There is one “bug” so to speak, and that it will potentially display garbage when applied to non-ascii text, such as Japanese.

Finally, these code snippets are GPL 2 or 3 at your convenience.