'Tests the node boundary of a directed graph.'

def test_directed(self):

    G = nx.DiGraph([(0, 1), (1, 2), (2, 3), (3, 4), (4, 0)])
    S = {0, 1}
    boundary = nx.node_boundary(G, S)
    expected = {2}
    assert_equal(boundary, expected)
