docs: def add_filtered_folder(self, path, regex, depth=None, source_type=DefaultSourceType):
          """
          Add a folder source to scan recursively, with a regex filter on directories.
  
          :param regex: regex string to filter folders by.
          :param depth: if provided will be depth limit. 0 = first level only.
          :param source_type: what to return; files only, folders only, or both.
          """
          self.add_source(FilteredFolderSource(path, regex, depth, **source_type))
          return self