sicp-ex-3.16

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 (count-pairs (list 'a 'b 'c))  ;; => 3 
  
 (define second (cons 'a 'b)) 
 (define third (cons 'a 'b)) 
 (define first (cons second third)) 
 (set-car! third second) 
 (count-pairs first)  ;; => 4 
  
 (define third (cons 'a 'b)) 
 (define second (cons third third)) 
 (define first (cons second second)) 
 (count-pairs first)  ;; => 7 
  
 (define lst (list 'a 'b 'c)) 
 (set-cdr! (cddr lst) lst) 
 (count-pairs lst)  ;; never returns