public function copyFrom ($data) {
        if (is_array ($data) || ($data instanceof \Traversable)) {
            $this->clear ();
            foreach ($data as $item) {
                $this->_d[] = $item;
                ++$this->_c;
            }
        } elseif ($data !== null)
            throw new Exception(\Yii::t ('yii' , 'Queue data must be an array or an object implementing Traversable.'));
    }