assertThat(network.edgesConnecting(N1, N1)).isEmpty();
    assertThat(network.edgesConnecting(N1, N2)).containsExactly(E12);
  }

  @Test
  public void concurrentIteration() throws Exception {
    addEdge(1, 2, "foo");
    addEdge(3, 4, "bar");
    addEdge(5, 6, "baz");

    int threadCount = 20;
    ExecutorService executor = newFixedThreadPool(threadCount);