def has_quorum(self):
        """
        we've seen +2/3 of all eligible votes voting for one block.
        there is a quorum.
        """
        assert self.is_valid
        bhs = self.blockhashes()
        if bhs and bhs[0][1] > 2 / 3. * self.num_eligible_votes:
            return bhs[0][0]