code: public void setRoot(int root)
    {
      super.setRoot(root);
      m_iterator = getDTM(root).getAxisIterator(m_axis);
      m_iterator.setStartNode(root);
    }
docs: Set the root node of the TreeWalker.
  (Not part of the DOM2 TreeWalker interface).
  
  @param root The context node of this step.