docs: def grouped_items(self, folder_id, subfolder_id, ann_id=None):
          '''Returns a dictionary from content ids to subtopic ids.
  
          Namely, the mapping is ``content_id |--> list of subtopic id``.
  
          By default (with ``ann_id=None``), subfolders are shown for all
          anonymous users. Optionally, ``ann_id`` can be set to a username,
          which restricts the list to only subfolders owned by that user.
  
          :param str folder_id: Folder id
          :param str subfolder_id: Subfolder id
          :param str ann_id: Username
          :rtype: ``dict`` of ``content_id |--> [subtopic_id]``
          '''
          d = defaultdict(list)
          for cid, subid in self.items(folder_id, subfolder_id, ann_id=ann_id):
              d[cid].append(subid)
          return d