docs: def is_partial(self, filename):
          """Check if a file is a partial.
  
          Partial files are not rendered, but they are used in rendering
          templates.
  
          A file is considered a partial if it or any of its parent directories
          are prefixed with an ``'_'``.
  
          :param filename: the name of the file to check
          """
          return any((x.startswith("_") for x in filename.split(os.path.sep)))