Exercise 5.28.  Modify the definition of the evaluator by changing 
eval-sequence as described in section 5.4.2 so that the evaluator is no 
longer tail-recursive. Rerun your experiments from exercises 5.26 and 
5.27 to demonstrate that both versions of the factorial procedure now 
require space that grows linearly with their input.

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Code in 5.28-eceval.scm, with ev-sequence changes as given in the text.

This version would be iterative, but without tail recursion it is not:

;;; EC-Eval input:
(define (factorial n)
  (define (iter out n)
    (if (= n 1) out (iter (* out n) (- n 1))))
  (iter 1 n))

(factorial 3)
(total-pushes = 107 maximum-depth = 20)
6

(factorial 4)
(total-pushes = 144 maximum-depth = 23)
24

(factorial 5)
(total-pushes = 181 maximum-depth = 26)
120


The recursive version is still recursive, but with slightly higher 
values for pushes and depth.

;;; EC-Eval input:
(define (factorial n)
  (if (= n 1) 1 (* n (factorial (- n 1)))))

(factorial 3)
(total-pushes = 86 maximum-depth = 23)
6

(factorial 4)
(total-pushes = 120 maximum-depth = 29)
24

(factorial 5)
(total-pushes = 154 maximum-depth = 35)
120


So, as expected, the change has made the iterative version run in linear 
space, and made code that was already recursive do a little extra work.