public function createSkipListIndex($collectionId, array $fields, $unique = null, $sparse = null)
    {
        $indexOptions = [];

        if ($unique) {
            $indexOptions[self::OPTION_UNIQUE] = (bool) $unique;
        }
        if ($sparse) {
            $indexOptions[self::OPTION_SPARSE] = (bool) $sparse;
        }

        return $this->index($collectionId, self::OPTION_SKIPLIST_INDEX, $fields, null, $indexOptions);
    }