@CanIgnoreReturnValue
  public static <T extends @Nullable Object> boolean addAll(
      Collection<T> addTo, Iterable<? extends T> elementsToAdd) {
    if (elementsToAdd instanceof Collection) {
      Collection<? extends T> c = (Collection<? extends T>) elementsToAdd;
      return addTo.addAll(c);
    }
    return Iterators.addAll(addTo, checkNotNull(elementsToAdd).iterator());
  }

  /**

   */
  @CanIgnoreReturnValue
  public static <T extends @Nullable Object> boolean addAll(
      Collection<T> addTo, Iterator<? extends T> iterator) {
    checkNotNull(addTo);
    checkNotNull(iterator);
    boolean wasModified = false;
    while (iterator.hasNext()) {
      wasModified |= addTo.add(iterator.next());
    }
    return wasModified;
  }

  /**

   */
  private static <E extends @Nullable Object> boolean addAllImpl(
      Multiset<E> self, AbstractMapBasedMultiset<? extends E> elements) {
    if (elements.isEmpty()) {
      return false;
    }
    elements.addTo(self);
    return true;
  }

  /** An implementation of {@link Multiset#removeAll}. */
  static boolean removeAllImpl(Multiset<?> self, Collection<?> elementsToRemove) {
    Collection<?> collection =