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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 convert the MNIST dataset files. Parameters subparser : :class:`argparse.ArgumentParser` Subparser handling the `mnist` command. """

    subparser.add_argument(
        "--dtype", help="dtype to save to; by default, images will be " +
        "returned in their original unsigned byte format",
        choices=('float32', 'float64', 'bool'), type=str, default=None)
    return convert_mnist