* allocate an object for each entry, so we check for that.
     */
    static <T> @Nullable ObjectCountHashMap<T> tryGetMap(Iterable<T> multiset) {
      if (multiset instanceof RegularImmutableMultiset) {
        return ((RegularImmutableMultiset<T>) multiset).contents;
      } else if (multiset instanceof AbstractMapBasedMultiset) {
        return ((AbstractMapBasedMultiset<T>) multiset).backingMap;
      } else {

   */
  @SuppressWarnings("unchecked") // all supported methods are covariant
  public static <E> ImmutableMultiset<E> of() {
    return (ImmutableMultiset<E>) RegularImmutableMultiset.EMPTY;
  }

  /**
   * Returns an immutable multiset containing a single element.
   *
   * @throws NullPointerException if the element is null
   * @since 6.0 (source-compatible since 2.0)
   */