docs: def add_item(self, folder_id, subfolder_id, content_id, subtopic_id=None,
                   ann_id=None):
          '''Add an item to a subfolder.
  
          The format of ``content_id`` and ``subtopic_id`` is
          unspecified. It is application specific.
  
          If ``ann_id`` is set, then the item is owned by the given user.
          Otherwise, the item is owned and viewable by all anonymous
          users.
  
          :param str folder_id: Folder id
          :param str subfolder_id: Folder id
          :param str content_id: content identifier
          :param str subtopic_id: subtopic identifier
          :param str ann_id: Username
          '''
          self.assert_valid_folder_id(folder_id)
          self.assert_valid_folder_id(subfolder_id)
          ann_id = self._annotator(ann_id)
          folder_cid = self.wrap_folder_content_id(ann_id, folder_id)
          subfolder_sid = self.wrap_subfolder_subtopic_id(subfolder_id)
  
          if self.store.get(folder_cid) is None:
              raise KeyError(folder_id)
  
          lab = Label(folder_cid, content_id,
                      ann_id, CorefValue.Positive,
                      subtopic_id1=subfolder_sid,
                      subtopic_id2=subtopic_id)
          self.label_store.put(lab)
          logger.info('Added subfolder item: %r', lab)