e = assertThrows(IllegalArgumentException.class, () -> directedGraph.addEdge(N1, N1, E11));
    assertThat(e).hasMessageThat().contains(ERROR_SELF_LOOP);
  }

  @Test
  public void createUndirected() {
    MutableNetwork<Integer, String> undirectedGraph = NetworkBuilder.undirected().build();
    assertThat(undirectedGraph.nodes()).isEmpty();
    assertThat(undirectedGraph.edges()).isEmpty();