docs: def get_cts_metadata(self, key: str, lang: str = None) -> Literal:
          """ Get easily a metadata from the CTS namespace
  
          :param key: CTS property to retrieve
          :param lang: Language in which it should be
          :return: Literal value of the CTS graph property
          """
          return self.metadata.get_single(RDF_NAMESPACES.CTS.term(key), lang)