(defun morris-counter ()
  "Returns a closure which accepts two messages
     :PLUS - probabilistically increments counter
     :SIZE - returns approximate # of times :PLUS was sent
   Any other message raises an error."
  (let ((c 0))
    (lambda (msg)
      (ecase msg
        (:plus (when (zerop (random (expt 2 c)))
                 (incf c)))
        (:size (1- (expt 2 c)))))))
 
(defvar *mc* (morris-counter))
(dotimes (i 12 t) (funcall *mc* :plus))
(pprint (funcall *mc* :size))

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