assertThrows(
        IllegalArgumentException.class,
        () -> inducedSubgraph(undirectedGraph, ImmutableSet.of(N1)));
  }

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

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