docs: def add_folder(self, folder_id, ann_id=None):
          '''Add a folder.
  
          If ``ann_id`` is set, then the folder is owned by the given user.
          Otherwise, the folder is owned and viewable by all anonymous
          users.
  
          :param str folder_id: Folder id
          :param str ann_id: Username
          '''
          self.assert_valid_folder_id(folder_id)
          ann_id = self._annotator(ann_id)
          cid = self.wrap_folder_content_id(ann_id, folder_id)
          self.store.put([(cid, FeatureCollection())])
          logger.info('Added folder %r with content id %r', folder_id, cid)