*
   * <p>The default value is {@code false}.
   */
  @CanIgnoreReturnValue
  public NetworkBuilder<N, E> allowsParallelEdges(boolean allowsParallelEdges) {
    this.allowsParallelEdges = allowsParallelEdges;
    return this;
  }

  /**
   * Specifies whether the network will allow self-loops (edges that connect a node to itself).

    String networkString = network.toString();
    assertThat(networkString).contains("isDirected: " + network.isDirected());
    assertThat(networkString).contains("allowsParallelEdges: " + network.allowsParallelEdges());
    assertThat(networkString).contains("allowsSelfLoops: " + network.allowsSelfLoops());

    int nodeStart = networkString.indexOf("nodes:");
    int edgeStart = networkString.indexOf("edges:");

   * 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

    // In a directed graph, parallel edges cannot introduce a cycle in an acyclic graph.
    // However, in an undirected graph, any parallel edge induces a cycle in the graph.
    if (!network.isDirected()
        && network.allowsParallelEdges()
        && network.edges().size() > network.asGraph().edges().size()) {
      return true;
    }
    return hasCycle(network.asGraph());
  }

  /**

    // In a directed graph, parallel edges cannot introduce a cycle in an acyclic graph.
    // However, in an undirected graph, any parallel edge induces a cycle in the graph.
    if (!network.isDirected()
        && network.allowsParallelEdges()
        && network.edges().size() > network.asGraph().edges().size()) {
      return true;
    }
    return hasCycle(network.asGraph());
  }

  /**