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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 ILSVRC2012 dataset files. Note that you will need to use `--url-prefix` to download the non-public files (namely, the TARs of images). This is a single prefix that is common to all distributed files, which you can obtain by registering at the ImageNet website [DOWNLOAD]. Note that these files are quite large and you may be better off simply downloading them separately and running ``fuel-convert``. .. [DOWNLOAD] http://www.image-net.org/download-images Parameters subparser : :class:`argparse.ArgumentParser` Subparser handling the `ilsvrc2012` command. """

    urls = ([None] * len(ALL_FILES))
    filenames = list(ALL_FILES)
    subparser.set_defaults(urls=urls, filenames=filenames)
    subparser.add_argument('-P', '--url-prefix', type=str, default=None,
                           help="URL prefix to prepend to the filenames of "
                                "non-public files, in order to download them. "
                                "Be sure to include the trailing slash.")
    return default_downloader