def copy(self):
        """Return a shallow copy of this graph.
        """
        other = DirectedGraph()
        other._vertices = set(self._vertices)
        other._forwards = {k: set(v) for k, v in self._forwards.items()}
        other._backwards = {k: set(v) for k, v in self._backwards.items()}
        return other
Return a shallow copy of this graph.