collector.lisp

(defun collector ()
  (let ((tail nil)
        (head nil))
    (lambda (&optional (item nil item?))
      (cond
        ((not item?) head)
        ((null tail) (setq tail (cons item nil)
                           head tail))
        (t (let ((new-tail (cons item nil)))
             (rplacd tail new-tail)
             (setq tail new-tail)))))))

(let ((collect (collector)))
  (dotimes (i 10)
    (funcall collect i))
  (funcall collect))