anchor.diff:                      (make-instance 'x-tree-node
                                      :children (list (root-node tree) new))))
                  (setf (parent (root-node tree)) new-root
  -                     (split-tree new-root) (cons (split-tree (root-node tree))
  -                                                 (split-tree new))
  +                     (split-tree new-root) (cons (root-node tree) new)
                        (root-node tree) new-root
                        (parent new) new-root)))))))
       tree))
  
positive.code:     ((atom split-tree) (list split-tree))
      (t (append (leaves-of-split-tree (car split-tree))
                 (leaves-of-split-tree (cdr split-tree))))))
  
  ;;; Figure 7: X-tree Insertion Algorithm for Directory Nodes
  (defmethod adjust-tree ((tree x-tree) node &optional new)
  
negative.code: 
  (defclass x-tree-node (spatial-tree-node)
    ((split-tree :initarg :split-tree :accessor split-tree)))
  
  ;;; Do we actually need to keep track of what is and what isn't a
  ;;; supernode?  I'm not sure that we do...
  (defclass x-tree-supernode (x-tree-node)
  