Seen this object (known by its id()?  Return 1 if so,
        otherwise add it to our memory and return 0.

def Known(self, obj):
        '''Seen this object (known by its id()?  Return 1 if so,
        otherwise add it to our memory and return 0.
        '''
        obj = _get_idstr(obj)
        if obj in self.memo: return 1
        self.memo.append(obj)
        return 0