def hr(self):
        """compute (height,round)
        We might have multiple rounds before we see consensus for a certain height.
        If everything is good, round should always be 0.
        """
        assert len(self), 'no votes, can not determine height'
        h = set([(v.height, v.round) for v in self.votes])
        assert len(h) == 1, len(h)
        return h.pop()