undirectedGraph.putEdgeValue(N1, N2, E12);

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

  @Test
  public void transpose_directedValueGraph() {
    MutableValueGraph<Integer, String> directedGraph =
        ValueGraphBuilder.directed().allowsSelfLoops(true).build();
    directedGraph.putEdgeValue(N1, N3, E13);