code: def _download_query(self, as_of):
          
          c = self.institution.client()
          q = c.brokerage_account_query(
              number=self.number, date=as_of, broker_id=self.broker_id)
          return q
docs: Formulate the specific query needed for download
  
          Not intended to be called by developers directly.
  
          :param as_of: Date in 'YYYYMMDD' format
          :type as_of: string