assertThat(e).hasMessageThat().contains(ERROR_REUSE_EDGE);
  }

  @Test
  public void addEdge_parallelEdge_notAllowed() {
    assume().that(graphIsMutable()).isTrue();
    assume().that(network.allowsParallelEdges()).isFalse();

    addEdge(N1, N2, E12);
    IllegalArgumentException e =
        assertThrows(
            IllegalArgumentException.class,

   * an edge connecting node A to node B if this {@link Network} has an edge connecting A to B.
   *
   * <p>If this network {@link #allowsParallelEdges() allows parallel edges}, parallel edges will be
   * treated as if collapsed into a single edge. For example, the {@link #degree(Object)} of a node

      GraphBuilder.directed().<String>immutable().addNode("B").build();

  private static final NetworkBuilder<?, ?> networkBuilderA =
      NetworkBuilder.directed().allowsParallelEdges(true).expectedNodeCount(10);
  private static final NetworkBuilder<?, ?> networkBuilderB =
      NetworkBuilder.directed().allowsSelfLoops(true).expectedNodeCount(16);

    Random gen = new Random(42); // Fixed seed so test results are deterministic.

    for (int trial = 0; trial < NUM_TRIALS; ++trial) {
      MutableNetwork<Integer, Object> network =
          networkBuilder.allowsParallelEdges(true).allowsSelfLoops(true).build();

      assertThat(network.nodes()).isEmpty();
      assertThat(network.edges()).isEmpty();
      AbstractNetworkTest.validateNetwork(network);

    assertThat(graph.allowsSelfLoops()).isEqualTo(asNetwork.allowsSelfLoops());
    assertThat(asNetwork.edgeOrder()).isEqualTo(ElementOrder.unordered());
    assertThat(asNetwork.allowsParallelEdges()).isFalse();
    assertThat(asNetwork.asGraph()).isEqualTo(graph);

    Set<EndpointPair<N>> allEndpointPairs = new HashSet<>();

    for (N node : sanityCheckSet(graph.nodes())) {

      }

      @Override
      public boolean isDirected() {
        return AbstractBaseGraph.this.isDirected();
      }

      @Override
      public boolean allowsParallelEdges() {
        return false; // Graph doesn't allow parallel edges
      }

      @Override
      public boolean allowsSelfLoops() {
        return AbstractBaseGraph.this.allowsSelfLoops();
      }

    assertThat(graph.allowsSelfLoops()).isEqualTo(asNetwork.allowsSelfLoops());
    assertThat(asNetwork.edgeOrder()).isEqualTo(ElementOrder.unordered());
    assertThat(asNetwork.allowsParallelEdges()).isFalse();
    assertThat(asNetwork.asGraph()).isEqualTo(graph.asGraph());

    for (Integer node : graph.nodes()) {
      assertThat(graph.adjacentNodes(node)).isEqualTo(asGraph.adjacentNodes(node));

    assertThat(graph.allowsSelfLoops()).isEqualTo(asNetwork.allowsSelfLoops());
    assertThat(asNetwork.edgeOrder()).isEqualTo(ElementOrder.unordered());
    assertThat(asNetwork.allowsParallelEdges()).isFalse();
    assertThat(asNetwork.asGraph()).isEqualTo(graph);

    Set<EndpointPair<N>> allEndpointPairs = new HashSet<>();

    for (N node : sanityCheckSet(graph.nodes())) {

    assertThat(graph.allowsSelfLoops()).isEqualTo(asNetwork.allowsSelfLoops());
    assertThat(asNetwork.edgeOrder()).isEqualTo(ElementOrder.unordered());
    assertThat(asNetwork.allowsParallelEdges()).isFalse();
    assertThat(asNetwork.asGraph()).isEqualTo(graph.asGraph());

    for (Integer node : graph.nodes()) {
      assertThat(graph.adjacentNodes(node)).isEqualTo(asGraph.adjacentNodes(node));