code: public void setElementAt(Node node, int index)
    {
      if (!m_mutable)
        throw new RuntimeException(XSLMessages.createXPATHMessage(XPATHErrorResources.ER_NODESET_NOT_MUTABLE, null)); //"This NodeSet is not mutable!");
  
      if (null == m_map)
      {
        m_map = new Node[m_blocksize];
        m_mapSize = m_blocksize;
      }
  
      m_map[index] = node;
    }
docs: Sets the component at the specified index of this vector to be the
  specified object. The previous component at that position is discarded.
  
  The index must be a value greater than or equal to 0 and less
  than the current size of the vector.
  
  @param node Node to set
  @param index Index of where to set the node