public function clear()
    {
        if (null !== $this->_nextScrollId) {
            $this->_search->getClient()->request(
                '_search/scroll',
                Request::DELETE,
                [Search::OPTION_SCROLL_ID => [$this->_nextScrollId]]
            );

            // Reset scroll ID so valid() returns false.
            $this->_nextScrollId = null;
            $this->_currentResultSet = null;
        }
    }