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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 the MNIST dataset files. The following MNIST dataset files are downloaded from Yann LeCun's website [LECUN]: `train-images-idx3-ubyte.gz`, `train-labels-idx1-ubyte.gz`, `t10k-images-idx3-ubyte.gz`, `t10k-labels-idx1-ubyte.gz`. Parameters subparser : :class:`argparse.ArgumentParser` Subparser handling the `mnist` command. """

    filenames = ['train-images-idx3-ubyte.gz', 'train-labels-idx1-ubyte.gz',
                 't10k-images-idx3-ubyte.gz', 't10k-labels-idx1-ubyte.gz']
    urls = ['http://yann.lecun.com/exdb/mnist/' + f for f in filenames]
    subparser.set_defaults(urls=urls, filenames=filenames)
    return default_downloader