1 (defun good-count-pairs (x)
2   (let ((pairs-table (make-hash-table :test #'eq)))
3     (labels (
4 	     (traverse-count (x)
5 	       (cond
6 		 ((not (consp x)) 0)
7 		 ((gethash x pairs-table) 0)
8 		 (t
9 		  (setf (gethash x pairs-table) 1)
10 		  (+ (traverse-count (car x))
11 		     (traverse-count (cdr x))
12 		     1)))))
13       (traverse-count x))))