* @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) {