public function getImplementationByContent(core_kernel_classes_Resource $content)
    {
        foreach ($content->getTypes() as $type) {
            if ($type->isSubClassOf(new core_kernel_classes_Class(DeliveryContent::CLASS_URI))) {
                return $this->getImplementationByContentClass($type);
            }
        }
        throw new common_exception_NoImplementation('No implementation found for DeliveryContent ' . $content->getUri());
    }