NetworkBuilder<? super N, ? super E> builder,
      Map<N, NetworkConnections<N, E>> nodeConnections,
      Map<E, N> edgeToReferenceNode) {
    this.isDirected = builder.directed;
    this.allowsParallelEdges = builder.allowsParallelEdges;
    this.allowsSelfLoops = builder.allowsSelfLoops;
    this.nodeOrder = builder.nodeOrder.cast();
    this.edgeOrder = builder.edgeOrder.cast();

          + "edgesConnecting() instead.";
  static final String PARALLEL_EDGES_NOT_ALLOWED =
      "Nodes %s and %s are already connected by a different edge. To construct a graph "
          + "that allows parallel edges, call allowsParallelEdges(true) on the Builder.";
  static final String SELF_LOOPS_NOT_ALLOWED =
      "Cannot add self-loop edge on node %s, as self-loops are not allowed. To construct a graph "

    return true;
  }

  private NetworkConnections<N, E> newConnections() {
    return isDirected()
        ? allowsParallelEdges()
            ? DirectedMultiNetworkConnections.<N, E>of()
            : DirectedNetworkConnections.<N, E>of()
        : allowsParallelEdges()
            ? UndirectedMultiNetworkConnections.<N, E>of()
            : UndirectedNetworkConnections.<N, E>of();
  }

  @Test
  public void equivalent_propertiesDiffer() {
    network.addEdge(N1, N2, E12);

    MutableNetwork<Integer, String> g2 =
        NetworkBuilder.from(network)
            .allowsParallelEdges(!network.allowsParallelEdges())
            .allowsSelfLoops(!network.allowsSelfLoops())
            .build();
    g2.addEdge(N1, N2, E12);

    assertThat(network).isEqualTo(g2);
  }

  @Test
  public void equivalent_propertiesDiffer() {
    network.addEdge(N1, N2, E12);

    MutableNetwork<Integer, String> g2 =
        NetworkBuilder.from(network)
            .allowsParallelEdges(!network.allowsParallelEdges())
            .allowsSelfLoops(!network.allowsSelfLoops())
            .build();
    g2.addEdge(N1, N2, E12);

    assertThat(network).isEqualTo(g2);
  }

  }

  /** Returns a string representation of this network. */
  @Override
  public String toString() {
    return "isDirected: "
        + isDirected()
        + ", allowsParallelEdges: "
        + allowsParallelEdges()
        + ", allowsSelfLoops: "
        + allowsSelfLoops()
        + ", nodes: "
        + nodes()
        + ", edges: "
        + edgeIncidentNodesMap(this);
  }

  /**

  }

  /** Returns a string representation of this network. */
  @Override
  public String toString() {
    return "isDirected: "
        + isDirected()
        + ", allowsParallelEdges: "
        + allowsParallelEdges()
        + ", allowsSelfLoops: "
        + allowsSelfLoops()
        + ", nodes: "
        + nodes()
        + ", edges: "
        + edgeIncidentNodesMap(this);
  }

  /**

    undirectedGraph = mutableUndirectedGraph;

    MutableNetwork<Integer, String> mutableDirectedNetwork =
        NetworkBuilder.directed().allowsParallelEdges(true).allowsSelfLoops(true).build();
    MutableNetwork<Integer, String> mutableUndirectedNetwork =
        NetworkBuilder.undirected().allowsParallelEdges(true).allowsSelfLoops(true).build();
    networksToTest = ImmutableList.of(mutableDirectedNetwork, mutableUndirectedNetwork);

   *     connect {@code nodeU} to {@code nodeV}
   * @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

  }

  @Test
  public void transpose_directedNetwork() {
    MutableNetwork<Integer, String> directedGraph =
        NetworkBuilder.directed().allowsParallelEdges(true).allowsSelfLoops(true).build();
    directedGraph.addEdge(N1, N3, E13);
    directedGraph.addEdge(N3, N1, E31);
    directedGraph.addEdge(N1, N2, E12);
    directedGraph.addEdge(N1, N2, E12_A);