assertThat(network.edgesConnecting("A", "B")).containsExactly("AB");
    assertThat(network.edgesConnecting("B", "A")).isEmpty();
  }

  @Test
  public void edgesConnecting_undirected() {
    MutableNetwork<String, String> mutableNetwork =
        NetworkBuilder.undirected().allowsSelfLoops(true).build();
    mutableNetwork.addEdge("A", "A", "AA");
    mutableNetwork.addEdge("A", "B", "AB");