public function setReturnEmpty($value)
    {
        if (is_bool($value)) {
            $this->_params['return-empty'] = ($value?'true':'false');
        } else if ($value != null) {
            $this->_params['return-empty'] = $value;
        } else {
            unset($this->_params['return-empty']);
        }
        return $this;
    }