docs: def reference_id(self, index):
          """Return the reference ID (locally defined)."""
          # TODO Include ISBN and ISSN?
          doi = self.reference_doi(index)
          if doi:
              return doi
          else:
              pmid = self.reference_pmid(index)
              if pmid:
                  return "PMID_" + pmid
              else:
                  unpublished_id = self.reference_unpublished_id(index)
                  if unpublished_id:
                      return "UNPUBLISHED_" + unpublished_id
          return ""