def unique_identifier(self):
        """
        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.
        """
        for t in IDENTIFIER_PRIORITY:
            found = self._tree.getroot().find('.//mods:identifier[@type="%s"]' % t, NS)
            if found is not None:
                return found.text
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.