Exercise 4.27.  Suppose we type in the following definitions to the lazy 
evaluator:

(define count 0)
(define (id x)
  (set! count (+ count 1))
  x)

Give the missing values in the following sequence of interactions, and 
explain your answers.

(define w (id (id 10)))
;;; L-Eval input:
count
;;; L-Eval value:
<response>
;;; L-Eval input:
w
;;; L-Eval value:
<response>
;;; L-Eval input:
count
;;; L-Eval value:
<response>

————————————————————————————————————————————————————————————————————————

(define w (id (id 10)))
;;; L-Eval input:
count
;;; L-Eval value:
0

Count is still 0 here because (id (id 10)) was dropped into a thunk 
which has not yet been forced, and id has not yet been called.


;;; L-Eval input:
w
;;; L-Eval value:
10

Now that the value of w is being printed to the screen, the evaluator 
must force it, which calls id (and increments count) twice.

;;; L-Eval input:
count
;;; L-Eval value:
2