* <p>Results are undefined if {@code multiset1} and {@code multiset2} are based on different
   * equivalence relations (as {@code HashMultiset} and {@code TreeMultiset} are).
   *
   * @since 14.0
   */
  public static <E extends @Nullable Object> Multiset<E> union(
      final Multiset<? extends E> multiset1, final Multiset<? extends E> multiset2) {
    checkNotNull(multiset1);
    checkNotNull(multiset2);

   * <p>Results are undefined if {@code multiset1} and {@code multiset2} are based on different
   * equivalence relations (as {@code HashMultiset} and {@code TreeMultiset} are).
   *
   * @since 14.0
   */
  public static <E extends @Nullable Object> Multiset<E> union(
      final Multiset<? extends E> multiset1, final Multiset<? extends E> multiset2) {
    checkNotNull(multiset1);
    checkNotNull(multiset2);

   */
  protected class StandardElementSet extends Multisets.ElementSet<E> {
    /** Constructor for use by subclasses. */
    public StandardElementSet() {}

    @Override
    Multiset<E> multiset() {
      return ForwardingMultiset.this;
    }

    @Override
    public Iterator<E> iterator() {
      return Multisets.elementIterator(multiset().entrySet().iterator());
    }
  }

  /**

          Multisets.immutableEntry(samples.e0(), 3),
          Multisets.immutableEntry(samples.e1(), 4),
          Multisets.immutableEntry(samples.e2(), 1),
          Multisets.immutableEntry(samples.e3(), 5),
          Multisets.immutableEntry(samples.e4(), 2));
    }

    @Override
    public Set<Multiset.Entry<E>> create(Object... entries) {
      List<Object> contents = new ArrayList<>();

   */
  protected class StandardElementSet extends Multisets.ElementSet<E> {
    /** Constructor for use by subclasses. */
    public StandardElementSet() {}

    @Override
    Multiset<E> multiset() {
      return ForwardingMultiset.this;
    }

    @Override
    public Iterator<E> iterator() {
      return Multisets.elementIterator(multiset().entrySet().iterator());
    }
  }

  /**

          Multisets.immutableEntry(samples.e0(), 3),
          Multisets.immutableEntry(samples.e1(), 4),
          Multisets.immutableEntry(samples.e2(), 1),
          Multisets.immutableEntry(samples.e3(), 5),
          Multisets.immutableEntry(samples.e4(), 2));
    }

    @Override
    public Set<Multiset.Entry<E>> create(Object... entries) {
      List<Object> contents = new ArrayList<>();

    return Collector.of(
        () -> TreeMultiset.create(comparator),
        (multiset, t) -> mapAndAdd(t, multiset, elementFunction, countFunction),
        (multiset1, multiset2) -> {
          multiset1.addAll(multiset2);
          return multiset1;
        },
        (Multiset<E> multiset) -> copyOfSortedEntries(comparator, multiset.entrySet()));
  }

  @SuppressWarnings("Java7ApiChecker")

  public void testCreation_noArgs() {
    Multiset<String> multiset = ImmutableSortedMultiset.of();
    assertTrue(multiset.isEmpty());
  }

  public void testCreation_oneElement() {
    Multiset<String> multiset = ImmutableSortedMultiset.of("a");
    assertEquals(HashMultiset.create(asList("a")), multiset);
  }

  public void testCreation_twoElements() {
    Multiset<String> multiset = ImmutableSortedMultiset.of("a", "b");

   * multiset for every occurrence in {@code c}, see {@link Multisets#retainOccurrences(Multiset,
   * Multiset)}.
   *
   * <p>This method refines {@link Collection#retainAll} to further specify that it <b>may not</b>
   * throw an exception in response to any of {@code elements} being null or of the wrong type.
   *
   * @see Multisets#retainOccurrences(Multiset, Multiset)
   */
  @CanIgnoreReturnValue

  public void testCreation_noArgs() {
    Multiset<String> multiset = ImmutableMultiset.of();
    assertTrue(multiset.isEmpty());
  }

  public void testCreation_oneElement() {
    Multiset<String> multiset = ImmutableMultiset.of("a");
    assertEquals(HashMultiset.create(asList("a")), multiset);
  }

  public void testCreation_twoElements() {
    Multiset<String> multiset = ImmutableMultiset.of("a", "b");