}
      return next;
    }
  }

  private static class ConcatenatedIterator<T extends @Nullable Object> implements Iterator<T> {
    /* The last iterator to return an element.  Calls to remove() go to this iterator. */
    @CheckForNull private Iterator<? extends T> toRemove;

    /* The iterator currently returning elements. */
    private Iterator<? extends T> iterator;

    /*

      }
      return next;
    }
  }

  private static class ConcatenatedIterator<T extends @Nullable Object> implements Iterator<T> {
    /* The last iterator to return an element.  Calls to remove() go to this iterator. */
    @CheckForNull private Iterator<? extends T> toRemove;

    /* The iterator currently returning elements. */
    private Iterator<? extends T> iterator;

    /*