}

    @Override
    @ParametricNullness
    public T next() {
      IndexedIterator<T> nextIndexed = queue.remove();
      PeekingIterator<T> nextIter = nextIndexed.iterator;
      T next = nextIter.next();
      if (nextIter.hasNext()) {
        queue.add(nextIndexed);
      }
      return next;
    }
  }