public function addTag(Dklab_Cache_Frontend_Tag $tag)
    {
        if ($tag->getBackend() !== $this->_getBackend()) {
            \Zend_Cache::throwException("Backends for tag " . get_class($tag) . " and slot " . get_class($this) . " must be same");
        }
        $this->_tags[] = $tag;
    }