protected function getIteratorByIndex($index = 0)
    {
        $runningCount = 0;
        foreach ($this->getArrayIterator() as $innerIt) {
            $count = count($innerIt);
            if ($index < $runningCount + $count) {
                return [$innerIt, $index - $runningCount];
            }
            $runningCount += $count;
        }

        return null;
    }