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