NetworkBuilder<N1, E1> castBuilder = cast();
    return new ImmutableNetwork.Builder<>(castBuilder);
  }

  /**
   * Specifies whether the network will allow parallel edges. Attempting to add a parallel edge to a
   * network that does not allow them will throw an {@link UnsupportedOperationException}.
   *
   * <p>The default value is {@code false}.
   */
  @CanIgnoreReturnValue

   * make it load its own version of FinalizableReferenceQueue. Then we need to interact with that
   * parallel version through reflection in order to exercise the parallel
   * FinalizableReferenceQueue, and then check that the parallel ClassLoader can be
   * garbage-collected after that.
   */

  public static class MyFinalizableWeakReference extends FinalizableWeakReference<Object> {

  // in one class (may be a utility class for error messages).
  private static final String ERROR_PARALLEL_EDGE = "connected by a different edge";
  private static final String ERROR_NEGATIVE_COUNT = "is non-negative";
  private static final String ERROR_ADDED_PARALLEL_EDGE =
      "Should not be allowed to add a parallel edge.";
  private static final String ERROR_ADDED_SELF_LOOP =
      "Should not be allowed to add a self-loop edge.";

   * make it load its own version of FinalizableReferenceQueue. Then we need to interact with that
   * parallel version through reflection in order to exercise the parallel
   * FinalizableReferenceQueue, and then check that the parallel ClassLoader can be
   * garbage-collected after that.
   */

  public static class MyFinalizableWeakReference extends FinalizableWeakReference<Object> {

/**
 * A class to represent the set of edges connecting an (implicit) origin node to a target node.
 *
 * <p>The {@link #outEdgeToNode} map allows this class to work on networks with parallel edges. See
 * {@link EdgesConnecting} for a class that is more efficient but forbids parallel edges.
 *
 * @author James Sexton
 * @param <E> Edge parameter type
 */
@ElementTypesAreNonnullByDefault
abstract class MultiEdgesConnecting<E> extends AbstractSet<E> {

   * 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 the {@link Graph} view may be less than the degree of the same node in this {@link Network}.
   */

    assertThat(list).containsExactly("a:1", "b:2", "c:3");
  }

  public void testForEachPair_parallel() {
    Stream<String> streamA = IntStream.range(0, 100000).mapToObj(String::valueOf).parallel();
    Stream<Integer> streamB = IntStream.range(0, 100000).mapToObj(i -> i).parallel();

    AtomicInteger count = new AtomicInteger(0);
    Streams.forEachPair(
        streamA,
        streamB,

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static com.google.common.graph.GraphConstants.PARALLEL_EDGES_NOT_ALLOWED;
import static com.google.common.graph.GraphConstants.REUSING_EDGE;
import static com.google.common.graph.GraphConstants.SELF_LOOPS_NOT_ALLOWED;
import static java.util.Objects.requireNonNull;

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import javax.annotation.CheckForNull;

/**
 * An implementation of {@link NetworkConnections} for undirected networks with parallel edges.
 *
 * @author James Sexton
 * @param <N> Node parameter type
 * @param <E> Edge parameter type
 */
@ElementTypesAreNonnullByDefault
final class UndirectedMultiNetworkConnections<N, E>

  /**
   * Returns a stream of a million primitive doubles. The stream is parallel, which should cause
   * {@code collect} calls to run in multithreaded mode, so testing the combiner as well as the
   * supplier and accumulator.
   */
  static DoubleStream megaPrimitiveDoubleStream() {
    return DoubleStream.iterate(0.0, x -> x + 1.0).limit(MEGA_STREAM_COUNT).parallel();
  }