gistfile1.txt

(defclass rbt-empty-node (red-black-node)
  ())

(defmethod (setf rbt-color) (new-value (obj rbt-empty-node))
  (error "Empty nodes can't be changed"))

(defmethod (setf left-child) (new-value (obj rbt-empty-node))
  (error "Empty nodes can't be changed"))

(defmethod (setf right-child) (new-value (obj rbt-empty-node))
  (error "Empty nodes can't be changed"))

(defmethod (setf parent) (new-value (obj rbt-empty-node))
  (error "Empty nodes can't be changed"))