docs: def subfolders(self, folder_id, ann_id=None):
          '''Yields an unodered generator of subfolders in a folder.
  
          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 ann_id: Username
          :rtype: generator of subfolder_id
          '''
          self.assert_valid_folder_id(folder_id)
          ann_id = self._annotator(ann_id)
          folder_cid = self.wrap_folder_content_id(ann_id, folder_id)
          if self.store.get(folder_cid) is None:
              raise KeyError(folder_id)
          all_labels = self.label_store.directly_connected(folder_cid)
          return nub(la.subtopic_for(folder_cid) for la in all_labels)