How to use the idutils.is_urn function in idutils

def add_uid(self, uid):
        """Add unique identifier in correct field."""
        # We might add None values from wherever. Kill them here.
        uid = uid or ''
        if _is_arxiv(uid):
            self._ensure_reference_field('arxiv_eprint', _normalize_arxiv(uid))
        elif idutils.is_doi(uid):
            self._ensure_reference_field('dois', [])
            self.obj['reference']['dois'].append(idutils.normalize_doi(uid))
        elif idutils.is_handle(uid):
            self._ensure_reference_field('persistent_identifiers', [])
            self.obj['reference']['persistent_identifiers'].append({
                'schema': 'HDL',
                'value': idutils.normalize_handle(uid),
            })
        elif idutils.is_urn(uid):
            self._ensure_reference_field('persistent_identifiers', [])
            self.obj['reference']['persistent_identifiers'].append({
                'schema': 'URN',
                'value': uid,
            })
        elif self.RE_VALID_CNUM.match(uid):
            self._ensure_reference_field('publication_info', {})
            self.obj['reference']['publication_info']['cnum'] = uid
        else:
            # idutils.is_isbn has a different implementation than normalize
            # isbn. Better to do it like this.
            try:
                isbn = idutils.normalize_isbn(uid)
                self._ensure_reference_field('isbn', {})
                self.obj['reference']['isbn'] = isbn.replace(' ', '').replace('-', '')
            # See https://github.com/nekobcn/isbnid/issues/2 and