code: public void setItem(int node, int index)
    {
  
      if (!m_mutable)
        throw new RuntimeException(XSLMessages.createXPATHMessage(XPATHErrorResources.ER_NODESETDTM_NOT_MUTABLE, null)); //"This NodeSetDTM is not mutable!");
  
      super.setElementAt(node, index);
    }
docs: Same as setElementAt.
  
  @param node  The node to be set.
  @param index The index of the node to be replaced.
  @throws RuntimeException thrown if this NodeSetDTM is not of
  a mutable type.