*         .allowsParallelEdges(true)
   *         .<City, Train>immutable()
   *         .addEdge(PARIS, BRUSSELS, Thalys.trainNumber("1111"))
   *         .addEdge(PARIS, BRUSSELS, RegionalTrain.trainNumber("2222"))
   *         .addEdge(LONDON, PARIS, Eurostar.trainNumber("3333"))
   *         .addEdge(LONDON, BRUSSELS, Eurostar.trainNumber("4444"))
   *         .addNode(REYKJAVIK)
   *         .build();
   * }

 * flightNetwork.addEdge("LAX", "ATL", 3025);
 * flightNetwork.addEdge("LAX", "ATL", 1598);
 * flightNetwork.addEdge("ATL", "LAX", 2450);
 *
 * // Building a immutable network
 * ImmutableNetwork<String, Integer> immutableNetwork =
 *     NetworkBuilder.directed()
 *         .allowsParallelEdges(true)
 *         .<String, Integer>immutable()
 *         .addEdge("LAX", "ATL", 3025)

   * @throws IllegalArgumentException if the introduction of the edge would violate {@link
   *     #allowsParallelEdges()} or {@link #allowsSelfLoops()}
   */
  @CanIgnoreReturnValue
  boolean addEdge(N nodeU, N nodeV, E edge);

  /**
   * Adds {@code edge} connecting {@code endpoints}. In an undirected network, {@code edge} will
   * also connect {@code nodeV} to {@code nodeU}.
   *

    graph.addNode(3);
    graph.addNode(1);
    graph.addNode(4);
  }

  private static void addEdges(MutableNetwork<Integer, String> network) {
    network.addEdge(3, 1, "i");
    network.addEdge(1, 4, "e");
    network.addEdge(4, 3, "p");
  }

  private static class NonComparableSuperClass {
    final Integer value;

    NonComparableSuperClass(Integer value) {

      for (E edge : network.outEdges(node)) {
        N successorNode = network.incidentNodes(edge).adjacentNode(node);
        if (subgraph.nodes().contains(successorNode)) {
          subgraph.addEdge(node, successorNode, edge);
        }
      }
    }
    return subgraph;
  }

  /** Creates a mutable copy of {@code graph} with the same nodes and edges. */