docs: def owned_objects(self):
          """
          List of gc-tracked objects owned by this ObjectGraph instance.
  
          """
          return (
              [
                  self,
                  self.__dict__,
                  self._head,
                  self._tail,
                  self._out_edges,
                  self._out_edges._keys,
                  self._out_edges._values,
                  self._in_edges,
                  self._in_edges._keys,
                  self._in_edges._values,
                  self._vertices,
                  self._vertices._elements,
                  self._edges,
              ] +
              list(six.itervalues(self._out_edges)) +
              list(six.itervalues(self._in_edges))
          )