code: private static <T> Iterator<Iterator<? extends T>> iterators(
        Iterable<? extends Iterable<? extends T>> iterables) {
      return new TransformedIterator<Iterable<? extends T>, Iterator<? extends T>>(
          iterables.iterator()) {
        @Override
        Iterator<? extends T> transform(Iterable<? extends T> from) {
          return from.iterator();
        }
      };
    }
docs: Returns an iterator over the iterators of the given iterables.