finds a round in the count history in which the candidate_ids each had different vote counts
        if no such round exists, returns None

def find_tie_breaker(self, candidate_ids):
        """
        finds a round in the count history in which the candidate_ids each had different vote counts
        if no such round exists, returns None
        """
        for candidate_aggregates in reversed(self.round_candidate_aggregates):
            candidates_on_vote = defaultdict(int)
            for candidate_id in candidate_ids:
                votes = candidate_aggregates.get_vote_count(candidate_id)
                candidates_on_vote[votes] += 1
            if max(candidates_on_vote.values()) == 1:
                return candidate_aggregates