undirectedGraph.putEdge(N1, N2);

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

  @Test
  public void transpose_directedGraph() {
    MutableGraph<Integer> directedGraph = GraphBuilder.directed().allowsSelfLoops(true).build();
    directedGraph.putEdge(N1, N3);
    directedGraph.putEdge(N3, N1);
    directedGraph.putEdge(N1, N2);