* @return a peeking iterator backed by that iterator. Apart from the additional {@link
   *     PeekingIterator#peek()} method, this iterator behaves exactly the same as {@code iterator}.
   */
  public static <T extends @Nullable Object> PeekingIterator<T> peekingIterator(
      Iterator<? extends T> iterator) {
    if (iterator instanceof PeekingImpl) {

   * @return a peeking iterator backed by that iterator. Apart from the additional {@link
   *     PeekingIterator#peek()} method, this iterator behaves exactly the same as {@code iterator}.
   */
  public static <T extends @Nullable Object> PeekingIterator<T> peekingIterator(
      Iterator<? extends T> iterator) {
    if (iterator instanceof PeekingImpl) {

    return CollectCollectors.toImmutableEnumSet();
  }

  /**
   * Returns a new, <i>mutable</i> {@code EnumSet} instance containing the given elements in their
   * natural order. This method behaves identically to {@link EnumSet#copyOf(Collection)}, but also
   * accepts non-{@code Collection} iterables and empty iterables.
   */
  public static <E extends Enum<E>> EnumSet<E> newEnumSet(