}
    };
  }

  /** Implementation of PeekingIterator that avoids peeking unless necessary. */
  private static class PeekingImpl<E extends @Nullable Object> implements PeekingIterator<E> {

    private final Iterator<? extends E> iterator;
    private boolean hasPeeked;
    @CheckForNull private E peekedElement;

    public PeekingImpl(Iterator<? extends E> iterator) {

      }
    };
  }

  /** Implementation of PeekingIterator that avoids peeking unless necessary. */
  private static class PeekingImpl<E extends @Nullable Object> implements PeekingIterator<E> {

    private final Iterator<? extends E> iterator;
    private boolean hasPeeked;
    @CheckForNull private E peekedElement;

    public PeekingImpl(Iterator<? extends E> iterator) {