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;
  }