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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 prompt_4_mfa_serial(input = None):
 """ Prompt for an MFA serial number :param input: Used for unit testing :return: The MFA serial number """

    return prompt_4_value('Enter your MFA serial:', required = False, regex = re_mfa_serial_format, regex_format = mfa_serial_format, input = input)