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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 fixed_string(self, data=None):
 """ The fixed string is used to identify a particular Yubikey device. The fixed string is referred to as the 'Token Identifier' in OATH-HOTP mode. The length of the fixed string can be set between 0 and 16 bytes. Tip: This can also be used to extend the length of a static password. """

        old = self.fixed
        if data != None:
            new = self._decode_input_string(data)
            if len(new) <= 16:
                self.fixed = new
            else:
                raise yubico_exception.InputError('The "fixed" string must be 0..16 bytes')
        return old