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)
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