'Test that pairs with no LCA specified emits nothing.'

def test_all_pairs_lowest_common_ancestor6(self):

    G = self.DG.copy()
    G.add_node((-1))
    gen = all_pairs_lca(G, [((-1), (-1)), ((-1), 0)])
    assert_equal(dict(gen), {((-1), (-1)): (-1)})
