def unique_identifier(self, purl):
        """
        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.
        """
        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
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.