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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def fill_subparser(subparser):
 """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. """

    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