(defun visit-in-order-helper (value visitor post-visited)
  (funcall visitor value)
  (when post-visited
    (funcall (caar post-visited) (cdar post-visited) visitor (cdr post-visited))))

(defun visit-in-order-2 (node visitor &optional post-visited)
  (cond
    (node
     (visit-in-order-2 (node-left node) visitor (list* (cons #'visit-in-order-helper  (node-value node))
                                                       (cons #'visit-in-order-2 (node-right node))
                                                       post-visited)))
    (post-visited
     (funcall (caar post-visited) (cdar post-visited) visitor (cdr post-visited)))))