code: def unique_identifier(self, purl):
          
          id_el = None
          for t in IDENTIFIER_PRIORITY:
              id_el = self._tree.getroot().find('.//mods:identifier[@type="%s"]' % t, NS)
              if id_el is not None:
                  break
          if id_el is None:
              mods = self._tree.getroot().find('.//mods:mods', NS)
              id_el = ET.SubElement(mods, TAG_MODS_IDENTIFIER)
              id_el.set('type', 'purl')
          id_el.text = purl
docs: Set the unique identifier by looking through ``mods:identifier``
  
          See `specs <https://ocr-d.github.io/mets#unique-id-for-the-document-processed>`_ for details.