code: public void insertElementAt(int value, int at)
    {
  
      if (!m_mutable)
        throw new RuntimeException(XSLMessages.createXPATHMessage(XPATHErrorResources.ER_NODESETDTM_NOT_MUTABLE, null)); //"This NodeSetDTM is not mutable!");
  
      super.insertElementAt(value, at);
    }
docs: Inserts the specified node in this vector at the specified index.
  Each component in this vector with an index greater or equal to
  the specified index is shifted upward to have an index one greater
  than the value it had previously.
  
  @param value The node to be inserted.
  @param at The index where the insert should occur.
  @throws RuntimeException thrown if this NodeSetDTM is not of
  a mutable type.