docs: def fs_cache(app_name='', cache_type='', idx=1,
               expires=DEFAULT_EXPIRES, cache_dir='', helper_class=_FSCacher):
      """
      A decorator to cache results of functions returning
      pd.DataFrame or pd.Series objects under:
      <cache_dir>/<app_name>/<cache_type>/<func_name>.<param_string>.csv,
      missing parts, like app_name and cache_type, will be omitted
  
      If cache_dir is omitted, stutils 'ST_FS_CACHE_PATH' conf dir will be used.
      If 'ST_FS_CACHE_PATH' is not configured, a temporary directory
      will be created.
  
      :param app_name: if present, cache files for this application will be
          stored in a separate folder
      :param idx: number of columns to use as an index
      :param cache_type: if present, cache files within app directory will be
          separated into different folders by their cache_type.
      :param expires: cache duration in seconds
      :param cache_dir: set custom file cache path
      """
  
      def decorator(func):
          return helper_class(func, cache_dir, app_name, cache_type, idx, expires)
  
      return decorator