topMetaIterator = checkNotNull(metaIterator);
    }

    // Returns a nonempty meta-iterator or, if all meta-iterators are empty, null.
    private @Nullable Iterator<? extends Iterator<? extends T>> getTopMetaIterator() {
      while (topMetaIterator == null || !topMetaIterator.hasNext()) {
        if (metaIterators != null && !metaIterators.isEmpty()) {
          topMetaIterator = metaIterators.removeFirst();
        } else {