addEdge(N1, N2, E12);
    assertThat(network.successors(N1)).containsExactlyElementsIn(successors);
  }

  @Test
  public void edges_containsOrderMismatch() {
    addEdge(N1, N2, E12);
    assertThat(network.asGraph().edges()).doesNotContain(ENDPOINTS_N2N1);
    assertThat(network.asGraph().edges()).doesNotContain(ENDPOINTS_N1N2);
  }

  @Test