code: def _format_url(instrument_type,
                  instrument_ids,
                  start_date,
                  end_date,
                  earliest_allowed_date):
      
      return (
          "http://www.bankofcanada.ca/stats/results/csv"
          "?lP=lookup_{instrument_type}_yields.php"
          "&sR={restrict}"
          "&se={instrument_ids}"
          "&dF={start}"
          "&dT={end}".format(
              instrument_type=instrument_type,
              instrument_ids='-'.join(map(prepend("L_"), instrument_ids)),
              restrict=earliest_allowed_date.strftime("%Y-%m-%d"),
              start=start_date.strftime("%Y-%m-%d"),
              end=end_date.strftime("%Y-%m-%d"),
          )
      )
docs: Format a URL for loading data from Bank of Canada.