/**
  * @param AbstractService $service
  * @return SpSsoDescriptor
  * @throws \InvalidArgumentException
  */
 public function addService(AbstractService $service)
 {
     $class = Helper::getClassNameOnly($service);
     if ($class != 'SingleLogoutService' && $class != 'SingleSignOnService') {
         throw new \InvalidArgumentException("Invalid service type {$class} for IDPSSODescriptor");
     }
     return parent::addService($service);
 }