public function export($type, $identifier, $version=null)
    {
        $model = $this->getPortableModelRegistry()->getModel($type);
        $object = $model->getRegistry()->fetch($identifier, $version);

        if (is_null($object)) {
            throw new PortableElementNotFoundException(
                'Unable to find a PCI associated to identifier: ' . $identifier
            );
        }
        $this->validate($object);
        return $model->getRegistry()->export($object);
    }