docs: def reference_year(self, index):
          """Return the reference publication year."""
          # TODO Use meta:parsedDate field instead?
          ref_date = self.reference_date(index)
          try:
              # NB: datetime.year returns an int.
              return parse(ref_date).year
          except ValueError:
              matched = re.search(r"\d{4}", ref_date)
              if matched:
                  return int(matched.group())
              else:
                  return ""