code: def fixed_string(self, data=None):
          
          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
docs: 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.