'Tests that the subgraph cannot change the graph structure'

def test_readonly(self):

    assert_raises(nx.NetworkXError, self.H.add_node, 5)
    assert_raises(nx.NetworkXError, self.H.remove_node, 0)
    assert_raises(nx.NetworkXError, self.H.add_edge, 5, 6)
    assert_raises(nx.NetworkXError, self.H.remove_edge, 0, 1)
