anchor.diff:  (defmethod split ((tree tree) old-child new-child-1 new-child-2)
     (setf (contents tree)
   	(make-instance (2-node-class tree)
  +	  :tree tree
   	  :parent tree
   	  :left new-child-1
   	  :right new-child-2)))
  
positive.code: 	   (split (parent node)
  		  node
  		  (make-instance (2-node-class (tree node))
  		    :parent parent
  		    :left (left node)
  		    :right (middle node))
  		  (make-instance (2-node-class (tree node))
  		    :parent parent
  		    :left new-child-1
  		    :right new-child-2))))))
  
negative.code: 	   (split (parent node)
  		  node
  		  (make-instance (2-node-class (tree node))
  		    :tree tree
  		    :parent parent
  		    :left (left node)
  		    :right new-child-1)
  		  (make-instance (2-node-class (tree node))
  		    :tree tree
  		    :parent parent
  		    :left new-child-1
  		    :right (right node))))
  