code: def find_tie_breaker(self, candidate_ids):
          
          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
docs: finds a round in the count history in which the candidate_ids each had different vote counts
          if no such round exists, returns None