'Tests that joining just one tree yields a tree with one more
node.'

def test_single(self):

    T = nx.empty_graph(1)
    actual = nx.join([(T, 0)])
    expected = nx.path_graph(2)
    assert_nodes_equal(list(expected), list(actual))
    assert_edges_equal(list(expected.edges()), list(actual.edges()))
