public function getSlot($key)
    {
        if (isset($this->_slots[$key])) {
            if (is_string($this->_slots[$key])) {
                return $this->_slots[$key];
            }
            ob_start();
            call_user_func($this->_slots[$key][0], $this->_slots[$key][1], $this->_slots[$key][2]);
            return ob_get_clean();
        } else if ($this->_parentContext) {
            return $this->_parentContext->getSlot($key);
        }
    }