public void copyOf_nullArgument() {
    assertThrows(NullPointerException.class, () -> copyOf((Graph<?>) null));
  }

  @Test
  public void copyOf_directedGraph() {
    Graph<Integer> directedGraph = buildDirectedGraph();

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

  @Test
  public void copyOf_undirectedGraph() {