public function mustRevalidate($enable = null)
    {
        deprecationWarning(
            'Response::mustRevalidate() is deprecated. ' .
            'Use withMustRevalidate() instead.'
        );

        if ($enable !== null) {
            if ($enable) {
                $this->_cacheDirectives['must-revalidate'] = true;
            } else {
                unset($this->_cacheDirectives['must-revalidate']);
            }
            $this->_setCacheControl();
        }

        return array_key_exists('must-revalidate', $this->_cacheDirectives);
    }