Exercise 3.9: In 1.2.1 we used the
substitution model to analyze two procedures for computing factorials, a
recursive version


(define (factorial n)
  (if (= n 1)
      1
      (* n (factorial (- n 1)))))

and an iterative version


(define (factorial n)
  (fact-iter 1 1 n))

(define (fact-iter product 
                   counter 
                   max-count)
  (if (> counter max-count)
      product
      (fact-iter (* counter product)
                 (+ counter 1)
                 max-count)))

Show the environment structures created by evaluating (factorial 6)
using each version of the factorial procedure.142