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: 	       (replace (parent node)
  			node
  			(make-instance 2-node-class
  			  :parent parent
  			  :left (left node)
  			  :right (make-instance 3-node-class
  				   :left (left middle)
  