code: public void setRoot(int context, Object environment)
    {
      super.setRoot(context, environment);
      if(m_axis > -1)
        m_iterator = m_cdtm.getAxisIterator(m_axis);
      m_iterator.setStartNode(m_context);
    }
docs: Initialize the context values for this expression
  after it is cloned.
  
  @param context The XPath runtime context for this
  transformation.