docs: def add_folder(self, path, depth=None, source_type=DefaultSourceType):
          """
          Add a folder source to scan recursively from path (string).
  
          :param path: folder path.
          :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(FolderSource(path, depth, **source_type))
          return self