public static <
          T extends @Nullable Object,
          K extends @Nullable Object,
          V extends @Nullable Object,
          M extends Multimap<K, V>>
      Collector<T, ?, M> flatteningToMultimap(
          java.util.function.Function<? super T, ? extends K> keyFunction,
          java.util.function.Function<? super T, ? extends Stream<? extends V>> valueFunction,

   */
  public static <
          T extends @Nullable Object,
          K extends @Nullable Object,
          V extends @Nullable Object,
          M extends Multimap<K, V>>
      Collector<T, ?, M> flatteningToMultimap(
          java.util.function.Function<? super T, ? extends K> keyFunction,
          java.util.function.Function<? super T, ? extends Stream<? extends V>> valueFunction,

 * implementation.
 *
 * @author Louis Wasserman
 */
@GwtIncompatible
public class MultimapTestSuiteBuilder<K, V, M extends Multimap<K, V>>
    extends PerCollectionSizeTestSuiteBuilder<
        MultimapTestSuiteBuilder<K, V, M>, TestMultimapGenerator<K, V, M>, M, Entry<K, V>> {

  public static <K, V, M extends Multimap<K, V>> MultimapTestSuiteBuilder<K, V, M> using(
      TestMultimapGenerator<K, V, M> generator) {

import org.jspecify.annotations.NonNull;
import org.jspecify.annotations.Nullable;

/** An ordering that treats {@code null} as less than all other values. */
@GwtCompatible
final class NullsFirstOrdering<T extends @Nullable Object> extends Ordering<@Nullable T>
    implements Serializable {
  final Ordering<? super T> ordering;

  NullsFirstOrdering(Ordering<? super T> ordering) {
    this.ordering = ordering;
  }

  @Override

import org.jspecify.annotations.NonNull;
import org.jspecify.annotations.Nullable;

/** An ordering that treats {@code null} as greater than all other values. */
@GwtCompatible
final class NullsLastOrdering<T extends @Nullable Object> extends Ordering<@Nullable T>
    implements Serializable {
  final Ordering<? super T> ordering;

  NullsLastOrdering(Ordering<? super T> ordering) {
    this.ordering = ordering;
  }

  @Override

import org.jspecify.annotations.NonNull;
import org.jspecify.annotations.Nullable;

/** An ordering that treats {@code null} as greater than all other values. */
@GwtCompatible
final class NullsLastOrdering<T extends @Nullable Object> extends Ordering<@Nullable T>
    implements Serializable {
  final Ordering<? super T> ordering;

  NullsLastOrdering(Ordering<? super T> ordering) {
    this.ordering = ordering;
  }

  @Override

     * operation O(1).
     */

    private @Nullable Iterator<? extends Iterator<? extends T>> topMetaIterator;

    // Only becomes nonnull if we encounter nested concatenations.
    private @Nullable Deque<Iterator<? extends Iterator<? extends T>>> metaIterators;

    ConcatenatedIterator(Iterator<? extends Iterator<? extends T>> metaIterator) {
      iterator = emptyIterator();

     * operation O(1).
     */

    private @Nullable Iterator<? extends Iterator<? extends T>> topMetaIterator;

    // Only becomes nonnull if we encounter nested concatenations.
    private @Nullable Deque<Iterator<? extends Iterator<? extends T>>> metaIterators;

    ConcatenatedIterator(Iterator<? extends Iterator<? extends T>> metaIterator) {
      iterator = emptyIterator();

 *
 * @author Hayward Chan
 */
final class Platform {
  static <K extends @Nullable Object, V extends @Nullable Object>
      Map<K, V> newHashMapWithExpectedSize(int expectedSize) {
    return Maps.newHashMapWithExpectedSize(expectedSize);
  }

  static <K extends @Nullable Object, V extends @Nullable Object>
      Map<K, V> newLinkedHashMapWithExpectedSize(int expectedSize) {

     */
    @CanIgnoreReturnValue
    @Override
    public Builder<E> addAll(Iterable<? extends E> elements) {
      requireNonNull(contents); // see the comment on the field
      if (elements instanceof Multiset) {
        Multiset<? extends E> multiset = (Multiset<? extends E>) elements;
        ObjectCountHashMap<? extends E> backingMap = tryGetMap(multiset);
        if (backingMap != null) {