code: def bulkref_hist(self, tickers, flds, dates, ovrds=None,
                       date_field='REFERENCE_DATE'):
          
          ovrds = [] if not ovrds else ovrds
  
          if type(tickers) is not list:
              tickers = [tickers]
          if type(flds) is not list:
              flds = [flds]
          self._send_hist(tickers, flds, dates, date_field, ovrds)
          data = self._parse_bulkref(flds, keep_corrId=True,
                                     sent_events=len(dates))
          data = pd.DataFrame(data)
          data.columns = ['ticker', 'field', 'name', 'value', 'position', 'date']
          data = data.sort_values(by=['date', 'position']).reset_index(drop=True)
          data = data.loc[:, ['date', 'ticker', 'field', 'name',
                              'value', 'position']]
          return data
docs: Make iterative calls to bulkref() and create a long DataFrame with
          columns [date, ticker, field, name, value, position] where each date
          corresponds to overriding a historical data override field.
  
          Parameters
          ----------
          tickers: {list, string}
              String or list of strings corresponding to tickers
          flds: {list, string}
              String or list of strings corresponding to FLDS
          dates: list
              list of date strings in the format YYYYmmdd
          ovrds: list of tuples
              List of tuples where each tuple corresponds to the override
              field and value. This should not include the date_field which will
              be iteratively overridden
          date_field: str
              Field to iteratively override for requesting historical data,
              e.g. REFERENCE_DATE, CURVE_DATE, etc.
  
          Example
          -------
          >>> import pdblp
          >>> con = pdblp.BCon()
          >>> con.start()
          >>> dates = ["20160625", "20160626"]
          >>> con.bulkref_hist("BVIS0587 Index", "CURVE_TENOR_RATES", dates,
          ...                  date_field="CURVE_DATE")