code: public void addNodesInDocOrder(NodeIterator iterator, XPathContext support)
    {
  
      if (!m_mutable)
        throw new RuntimeException(XSLMessages.createXPATHMessage(XPATHErrorResources.ER_NODESET_NOT_MUTABLE, null)); //"This NodeSet is not mutable!");
  
      Node node;
  
      while (null != (node = iterator.nextNode()))
      {
        addNodeInDocOrder(node, support);
      }
    }
docs: Copy NodeList members into this nodelist, adding in
  document order.  If a node is null, don't add it.
  
  @param iterator NodeIterator which yields the nodes to be added.
  @param support The XPath runtime context.
  @throws RuntimeException thrown if this NodeSet is not of
  a mutable type.