MutableNetwork<Integer, String> undirectedGraph =
        NetworkBuilder.undirected().allowsParallelEdges(true).allowsSelfLoops(true).build();
    undirectedGraph.addEdge(N1, N1, E11);
    undirectedGraph.addEdge(N1, N2, E12);
    undirectedGraph.addEdge(N1, N1, E11_A);
    undirectedGraph.addEdge(N1, N2, E12_A);
    undirectedGraph.addEdge(N2, N1, E21);

    return undirectedGraph;
  }

    assertThat(hasCycle(directedGraph)).isFalse();
    assertThat(hasCycle(undirectedGraph)).isFalse();
  }

  @Test
  public void hasCycle_isolatedNodes() {
    for (MutableGraph<Integer> graph : graphsToTest) {
      graph.addNode(1);
      graph.addNode(2);
    }
    assertThat(hasCycle(directedGraph)).isFalse();
    assertThat(hasCycle(undirectedGraph)).isFalse();
  }

  @Test
  public void hasCycle_oneEdge() {