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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 get_files(data):
 """Retrieve pre-installed viral reference files. """

    all_files = glob.glob(os.path.normpath(os.path.join(os.path.dirname(dd.get_ref_file(data)),
                                                        os.pardir, "viral", "*")))
    return sorted(all_files)