* supplied by the user of the class. For example, {@code Multiset.Entry.getElement()} returns
 * {@code @ParametricNullness E}, which means:
 *
 * <ul>
 *   <li>{@code getElement} on a {@code Multiset.Entry<@NonNull String>} returns {@code @NonNull
 *       String}.
 *   <li>{@code getElement} on a {@code Multiset.Entry<@Nullable String>} returns {@code @Nullable
 *       String}.
 * </ul>
 *

    private static final long serialVersionUID = 0;
  }

  static <E extends @Nullable Object> Multiset<E> multiset(
      Multiset<E> multiset, @CheckForNull Object mutex) {
    if (multiset instanceof SynchronizedMultiset || multiset instanceof ImmutableMultiset) {
      return multiset;
    }
    return new SynchronizedMultiset<>(multiset, mutex);
  }

  static final class SynchronizedMultiset<E extends @Nullable Object>

    private static final long serialVersionUID = 0;
  }

  static <E extends @Nullable Object> Multiset<E> multiset(
      Multiset<E> multiset, @CheckForNull Object mutex) {
    if (multiset instanceof SynchronizedMultiset || multiset instanceof ImmutableMultiset) {
      return multiset;
    }
    return new SynchronizedMultiset<>(multiset, mutex);
  }

  static final class SynchronizedMultiset<E extends @Nullable Object>

  }

  abstract Set<K> createKeySet();

  @LazyInit @CheckForNull private transient Multiset<K> keys;

  @Override
  public Multiset<K> keys() {
    Multiset<K> result = keys;
    return (result == null) ? keys = createKeys() : result;
  }

  abstract Multiset<K> createKeys();

  @LazyInit @CheckForNull private transient Collection<V> values;

  @Override

    HashMultisetImpl {
      @Override
      public <E extends Comparable<E>> Multiset<E> create(Collection<E> contents) {
        return HashMultiset.create(contents);
      }
    },
    LinkedHashMultisetImpl {
      @Override
      public <E extends Comparable<E>> Multiset<E> create(Collection<E> contents) {
        return LinkedHashMultiset.create(contents);
      }
    },

 * supplied by the user of the class. For example, {@code Multiset.Entry.getElement()} returns
 * {@code @ParametricNullness E}, which means:
 *
 * <ul>
 *   <li>{@code getElement} on a {@code Multiset.Entry<@NonNull String>} returns {@code @NonNull
 *       String}.
 *   <li>{@code getElement} on a {@code Multiset.Entry<@Nullable String>} returns {@code @Nullable
 *       String}.
 * </ul>
 *

 * supplied by the user of the class. For example, {@code Multiset.Entry.getElement()} returns
 * {@code @ParametricNullness E}, which means:
 *
 * <ul>
 *   <li>{@code getElement} on a {@code Multiset.Entry<@NonNull String>} returns {@code @NonNull
 *       String}.
 *   <li>{@code getElement} on a {@code Multiset.Entry<@Nullable String>} returns {@code @Nullable
 *       String}.
 * </ul>
 *

 * supplied by the user of the class. For example, {@code Multiset.Entry.getElement()} returns
 * {@code @ParametricNullness E}, which means:
 *
 * <ul>
 *   <li>{@code getElement} on a {@code Multiset.Entry<@NonNull String>} returns {@code @NonNull
 *       String}.
 *   <li>{@code getElement} on a {@code Multiset.Entry<@Nullable String>} returns {@code @Nullable
 *       String}.
 * </ul>
 *

 * supplied by the user of the class. For example, {@code Multiset.Entry.getElement()} returns
 * {@code @ParametricNullness E}, which means:
 *
 * <ul>
 *   <li>{@code getElement} on a {@code Multiset.Entry<@NonNull String>} returns {@code @NonNull
 *       String}.
 *   <li>{@code getElement} on a {@code Multiset.Entry<@Nullable String>} returns {@code @Nullable
 *       String}.
 * </ul>
 *

 * supplied by the user of the class. For example, {@code Multiset.Entry.getElement()} returns
 * {@code @ParametricNullness E}, which means:
 *
 * <ul>
 *   <li>{@code getElement} on a {@code Multiset.Entry<@NonNull String>} returns {@code @NonNull
 *       String}.
 *   <li>{@code getElement} on a {@code Multiset.Entry<@Nullable String>} returns {@code @Nullable
 *       String}.
 * </ul>
 *