'A tournament must have no self-loops.'

def test_self_loops(self):

    G = DiGraph()
    G.add_edges_from([(0, 1), (1, 2), (2, 3), (3, 0), (1, 3), (0, 2)])
    G.add_edge(0, 0)
    assert_false(is_tournament(G))
