Exercise 3.13: Consider the following
make-cycle procedure, which uses the last-pair procedure defined
in Exercise 3.12:


(define (make-cycle x)
  (set-cdr! (last-pair x) x)
  x)

Draw a box-and-pointer diagram that shows the structure z created by


(define z (make-cycle (list 'a 'b 'c)))

What happens if we try to compute (last-pair z)?