Graph<Integer> copy = copyOf(directedGraph);
    assertThat(copy).isEqualTo(directedGraph);
  }

  @Test
  public void copyOf_undirectedGraph() {
    Graph<Integer> undirectedGraph = buildUndirectedGraph();

    Graph<Integer> copy = copyOf(undirectedGraph);
    assertThat(copy).isEqualTo(undirectedGraph);
  }

  @Test
  public void copyOf_directedValueGraph() {