code: public void addNode(Node n)
    {
  
      if (!m_mutable)
        throw new RuntimeException(XSLMessages.createXPATHMessage(XPATHErrorResources.ER_NODESET_NOT_MUTABLE, null)); //"This NodeSet is not mutable!");
  
      this.addElement(n);
    }
docs: Add a node to the NodeSet. Not all types of NodeSets support this
  operation
  
  @param n Node to be added
  @throws RuntimeException thrown if this NodeSet is not of
  a mutable type.