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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 vault_file(env, default):
 """The path to a misc Vault file This function will check for the env override on a file path, compute a fully qualified OS appropriate path to the desired file and return it if it exists. Otherwise returns None """

    home = os.environ['HOME'] if 'HOME' in os.environ else \
        os.environ['USERPROFILE']
    filename = os.environ.get(env, os.path.join(home, default))
    filename = abspath(filename)
    if os.path.exists(filename):
        return filename

    return None