1 (defun has-loop? (x)
2   (let ((pairs-table (make-hash-table :test #'eq)))
3     (labels (
4 	     (traverse-list (x)
5 	       (cond
6 		 ((null x) nil)
7 		 ((gethash x pairs-table) t)
8 		 (t
9 		  (setf (gethash x pairs-table) 1)
10 		  (traverse-list (cdr x))))))
11       (traverse-list x))))