public function setRankingMode($ranker, $rankexpr = '')
    {
        assert($ranker === 0 || $ranker >= 1 && $ranker < SPH_RANK_TOTAL);
        assert(is_string($rankexpr));
        $this->_ranker = $ranker;
        $this->_rankexpr = $rankexpr;

        return $this;
    }