docs: def folders(self, ann_id=None):
          '''Yields an unordered generator for all available folders.
  
          By default (with ``ann_id=None``), folders are shown for all
          anonymous users. Optionally, ``ann_id`` can be set to a username,
          which restricts the list to only folders owned by that user.
  
          :param str ann_id: Username
          :rtype: generator of folder_id
          '''
          ann_id = self._annotator(ann_id)
          if len(self.prefix) > 0:
              prefix = '|'.join([urllib.quote(self.prefix, safe='~'),
                                 'topic', ann_id, ''])
          else:
              prefix = '|'.join(['topic', ann_id, ''])
          logger.info('Scanning for folders with prefix %r', prefix)
          return imap(lambda id: self.unwrap_folder_content_id(id)['folder_id'],
                      self.store.scan_prefix_ids(prefix))