code: public void addNodesInDocOrder(NodeList nodelist, XPathContext support)
    {
  
      if (!m_mutable)
        throw new RuntimeException(XSLMessages.createXPATHMessage(XPATHErrorResources.ER_NODESET_NOT_MUTABLE, null)); //"This NodeSet is not mutable!");
  
      int nChildren = nodelist.getLength();
  
      for (int i = 0; i < nChildren; i++)
      {
        Node node = nodelist.item(i);
  
        if (null != node)
        {
          addNodeInDocOrder(node, support);
        }
      }
    }
docs: Copy NodeList members into this nodelist, adding in
  document order.  If a node is null, don't add it.
  
  @param nodelist List of nodes to be added
  @param support The XPath runtime context.
  @throws RuntimeException thrown if this NodeSet is not of
  a mutable type.