1 (define (test-case actual expected)
2   (newline)
3   (display "Actual:   ")
4   (display actual)
5   (newline)
6   (display "Expected: ")
7   (display expected)
8   (newline))
9 
10 (define (last-pair x)
11   (if (null? (cdr x))
12       x
13       (last-pair (cdr x))))
14 
15 (define (make-cycle x)
16   (set-cdr! (last-pair x) x)
17   x)
18 
19 Draw a box-and-pointer diagram that shows the structure z created by
20 
21 (define z (make-cycle (list 'a 'b 'c)))
22 
23 What happens if we try to compute (last-pair z)?