Drupal Notifications API: notifications_user_get_subscriptions

This is the signature per Acquia's documentation:

notific开发者_如何学Pythonations_user_get_subscriptions($uid, $event_type, 
      $oid, $object = NULL, $refresh = FALSE)

I have used this method before to get node-level subscriptions for a user. I cannot figure out what to pass for parameter 4 if I want to get subscriptions for a specific content type only. This call returns all content type subscription:

notifications_user_get_subscriptions($author->uid, 'node', 'type');

What should parameter 4 be for this? The 'type' is simply 'forum' in the database, but passing the string 'forum' does not work.

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Until there is an answer to this question, I resorted to iterating through the results and testing the 'type' field. I posted the solution here.