1  Exercise 3.18.  Write a procedure that examines a list and determines whether it contains a cycle, that is, whether a program that tried to find the end of the list by taking successive cdrs would go into an infinite loop. Exercise 3.13 constructed such lists. 
2 
3 (define (test-case actual expected)
4   (newline)
5   (display "Actual:   ")
6   (display actual)
7   (newline)
8   (display "Expected: ")
9   (display expected)
10   (newline))
11 
12 (define (last-pair x)
13   (if (null? (cdr x))
14       x
15       (last-pair (cdr x))))
16 
17 (define (make-cycle x)
18   (set-cdr! (last-pair x) x)
19   x)
20 
21 (define (count-pairs x)
22   (if (not (pair? x))
23       0
24       (+ (count-pairs (car x))
25 	 (count-pairs (cdr x))
26 	 1)))
27 
28 (define three '(a b c))
29 (define a-pair (cons '() '()))
30 (define b-pair (cons a-pair a-pair))
31 (define four (cons 'a b-pair))
32 (define seven (cons b-pair b-pair))
33 (define circular (make-cycle '(a b c)))
34 
35 (test-case (count-pairs three) 3)
36 (test-case (count-pairs four) 4)
37 (test-case (count-pairs seven) 7)
38 (test-case (count-pairs circular) 'infinite-loop)