add an event or list of events that produces a predefined error
        an event is only added if its not already there

def faulty(self, *args):
        """
        add an event or list of events that produces a predefined error
        an event is only added if its not already there
        """
        for each in args:
            if not self.is_faulty(each):
                self._faults.add(each)