Exercise 5.21: Implement register machines for
the following procedures.  Assume that the list-structure memory operations are
available as machine primitives.


 Recursive count-leaves:


(define (count-leaves tree)
  (cond ((null? tree) 0)
        ((not (pair? tree)) 1)
        (else 
         (+ (count-leaves (car tree))
            (count-leaves (cdr tree))))))

 Recursive count-leaves with explicit counter:


(define (count-leaves tree)
  (define (count-iter tree n)
    (cond ((null? tree) n)
          ((not (pair? tree)) (+ n 1))
          (else 
           (count-iter 
            (cdr tree)
            (count-iter (car tree) 
                        n)))))
  (count-iter tree 0))