1 ;; Devise a correct version of the count-pairs procedure of exercise 3.16 that returns the number of distinct pairs in any structure. (Hint: Traverse the structure, maintaining an auxiliary data structure that is used to keep track of which pairs have already been counted.) 
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 (count-pairs x)
13   (if (not (pair? x))
14       0
15       (+ (count-pairs (car x))
16          (count-pairs (cdr x))
17          1)))
18 
19 (define three '(a b c))
20 (define four )
21 (define seven )
22 (define circular )
23