docs: def base_uri(self):
          """ Resolution base for JSON schema. Also used as the default
          graph ID for RDF. """
          if self._base_uri is None:
              if self._resolver is not None:
                  self._base_uri = self.resolver.resolution_scope
              else:
                  self._base_uri = 'http://pudo.github.io/jsongraph'
          return URIRef(self._base_uri)