code: def _get_hidden_forum_ids(self, forums, user):
          
          visible_forums = self._get_forums_for_user(
              user, ['can_see_forum', 'can_read_forum', ], use_tree_hierarchy=True,
          )
          return forums.exclude(id__in=[f.id for f in visible_forums])
docs: Given a set of forums and a user, returns the list of forums that are not visible by
              this user.