code: @Override
      public EventSubscription addSourceIdentifierToSubscription(AddSourceIdentifierToSubscriptionRequest request) {
          request = beforeClientExecution(request);
          return executeAddSourceIdentifierToSubscription(request);
      }
docs: <p>
  Adds a source identifier to an existing event notification subscription.
  </p>
  
  @param addSourceIdentifierToSubscriptionRequest
  @return Result of the AddSourceIdentifierToSubscription operation returned by the service.
  @throws SubscriptionNotFoundException
  @throws SourceNotFoundException
  @sample AmazonNeptune.AddSourceIdentifierToSubscription
  @see <a href="http://docs.aws.amazon.com/goto/WebAPI/neptune-2014-10-31/AddSourceIdentifierToSubscription"
  target="_top">AWS API Documentation</a>