anchor.diff:  ;;; Make CHILD the left child of NODE.
   (defgeneric link-left (parent child))
   
  -(defmethod link-left ((parent node) (child node))
  +(defmethod link-left ((parent node) child)
     (setf (left parent) child))
   
   (defmethod link-left ((parent node-with-parent) (child node-with-parent))
  
positive.code: ;;; Remove CHILD as the right child of PARENT.
  (defgeneric unlink-right (parent child))
  
  (defmethod unlink-right ((parent node) (child node))
    (setf (right parent) nil))
  
  (defmethod unlink-right ((parent node-with-parent) (child node-with-parent))
  
negative.code: ;;; Make CHILD the right child of PARENT.
  (defgeneric link-right (parent child))
  
  (defmethod link-right ((parent node) child)
    (setf (right parent) child))
  
  (defmethod link-right ((parent node-with-parent) (child node-with-parent))
  