code: def fill_subparser(subparser):
      
      subparser.add_argument(
          '--youtube-id', type=str, required=True,
          help=("The YouTube ID of the video from which to extract audio, "
                "usually an 11-character string.")
      )
      return download
docs: Sets up a subparser to download audio of YouTube videos.
  
      Adds the compulsory `--youtube-id` flag.
  
      Parameters
      ----------
      subparser : :class:`argparse.ArgumentParser`
          Subparser handling the `youtube_audio` command.