(define evaluate
       (lambda (expression-string)
	 (let ((expr-stack (make-ra-stack)))
	   (push! expr-stack '$)
	   (define process
	     (lambda (rest-of-expr)
	       ... scan the expression and take
		   action according to the action
		   table ...
	       ))
	   (process
	     (tokenize expression-string)))))