(defun cyc1 (a)
  (let ((ls (make-list 10 a)))
    (nconc ls ls)
    ls))

(ert-deftest test-cycle-assq ()
  (let ((c1 (cyc1 '(1))))
    (should-error (assq 3 c1) :type 'circular-list)))

(ert-deftest test-cycle-assoc ()
  (let ((c1 (cyc1 '(1))))
    (should-error (assoc 2 c1) :type 'circular-list)))

(defvar testcover-started nil)
(unless testcover-started
  (setq testcover-started t)
  (testcover-start (or load-file-name (buffer-file-name))))