code: def rule_special_registers(self, arg):
          
          # TODO is PSR supposed to be here?
          special_registers = "PSR APSR IPSR EPSR PRIMASK FAULTMASK BASEPRI CONTROL"
          if arg not in special_registers.split():
              raise iarm.exceptions.RuleError("{} is not a special register; Must be [{}]".format(arg, special_registers))
docs: Raises an exception if the register is not a special register