sicp exercise 3.20

; Exercise 3.20.  Draw environment diagrams to illustrate the evaluation of the sequence of expressions
;
; (define x (cons 1 2))
; (define z (cons x x))
; (set-car! (cdr z) 17)
; (car x)
; 17
;
; using the procedural implementation of pairs given above. (Compare exercise 3.11.)