anchor.diff:  (defmethod replace-and-shrink ((node 3-node) old-child new-child)
     (let ((2-node-class (2-node-class (tree node)))
   	(3-node-class (3-node-class (tree node)))
  +	(parent (parent node))
   	(left (left node))
   	(middle (middle node))
   	(right (right node)))
  
positive.code: 	       (replace (parent node)
  			node
  			(make-instance 2-node-class
  			  :left (left node)
  			  :right (make-instance 3-node-class
  				   :left (left middle)
  
negative.code: 	       (replace (parent node)
  			node
  			(make-instance 2-node-class
  			  :parent parent
  			  :left (make-instance 3-node-class
  				  :left (left left)
  				  :middle (right left)
  