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 (mystery x)
11   (define (loop x y)
12     (if (null? x)
13 	y
14 	(let ((temp (cdr x)))
15 	  (set-cdr! x y)
16 	  (loop temp x))))
17   (loop x '()))
18 
19 ;; Explain what mystery does in general.
20 
21 ;; mystery reverses the list x
22 
23 (define v (list 'a 'b 'c 'd))
24 
25 (define w (mystery v))
26 (test-case v '(a))
27 (test-case w '(d c b a))
28