undirectedGraph.addEdge(N1, N2, E12);

    assertThat(transpose(undirectedGraph)).isSameInstanceAs(undirectedGraph);
  }

  @Test
  public void transpose_directedNetwork() {
    MutableNetwork<Integer, String> directedGraph =
        NetworkBuilder.directed().allowsParallelEdges(true).allowsSelfLoops(true).build();
    directedGraph.addEdge(N1, N3, E13);