public function __isset($var)
    {
        if ($this->getContentInstance() instanceof ContentSet) {
            throw new UnknownPropertyException(sprintf(
                'Unknown property %s in %s.',
                $var,
                ClassUtils::getRealClass($this->getContentInstance())
            ));
        }

        return array_key_exists($var, $this->_data) && 0 < count($this->_data[$var]);
    }