(defun sim ()
  ;; strictly time based
  (let ((state '(1 1 0 1))              ;R S Q /Q
        (old '(x x x x))
        (ti 0))
    (labels ((show ()
               (apply #'format t "~& ~4D:  ~D ~D  ~D ~D~%"
                      ti state))
             (run ()
               (loop
                 (setq state (multiple-value-list (apply #'flip-flop state)))
                 (when (equal old state)
                   (return))
                 (setq old state)
                 (show) (incf ti))))
      ;;
      (run)
      (setf ti 5 (car state) 0)         ;R = 0
      (run)
      (setf ti 10 (car state) 1)        ;R = 1, again
      (run)
      (setf ti 15 (cadr state) 0)       ;S = 0
      (run)
      (setf ti 15 (cadr state) 1)       ;S = 1, again
      (run))))

(defun flip-flop (r s q /q)
  (psetq q (nand r /q)
         /q (nand s q))
  (values r s q /q))

(defun nand (x y)
  (logand 1 (lognand x y)))

;;  0:  1 1  0 1
;;  5:  0 1  1 1
;;  6:  0 1  1 0
;; 10:  1 1  1 0
;; 15:  1 0  1 1
;; 16:  1 0  0 1
;; 15:  1 1  0 1