code: public function addServiceDefinitionProvider(ServiceDefinitionProvider $provider) : Capabilities
      {
          if (! isset($this->serviceDefinitionProviders[$provider->handles()])) {
  
              $this->serviceDefinitionProviders[$provider->handles()] = $provider;
          }
  
          return $this;
      }
docs: @param ServiceDefinitionProvider $provider
  
  @return Capabilities