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