docs: def get_alternatives(self, rank):
          """
          Description:
              Returns the alternative(s) with the given ranking in the
              computed aggregate ranking.  An error is thrown if the
              ranking does not exist.  
          """
          if self.ranks_to_alts is None:
              raise ValueError("Aggregate ranking must be created first")
          try:
              alts = self.ranks_to_alts[rank]
              return alts
          except KeyError:
              raise KeyError("No ranking \"{}\" found in ".format(str(rank)) +
                             "the aggregate ranking")