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 (append! x y)
11   (set-cdr! (last-pair x) y)
12   x)
13 
14 (define (last-pair x)
15   (if (null? (cdr x))
16       x
17       (last-pair (cdr x))))
18 
19 (define x (list 'a 'b))
20 (define y (list 'c 'd))
21 (define z (append x y))
22 (test-case z '(a b c d))
23 (test-case (cdr x) '(b))
24 (define w (append! x y))
25 (test-case w '(a b c d))
26 (test-case (cdr x) '(b c d))