public function tac($flag = self::ADD_LAP)
    {
        $this->stop = microtime(true);
        if ($flag) {
            $this->elapsed = ($this->stop - $this->start) * 1e6
            - $this->calib;
        } else {
            $this->elapsed += ($this->stop - $this->start) * 1e6
            - $this->calib;
        }
        ++$this->count;
    }