Description:
            Returns the alternative(s) with the given ranking in the
            computed aggregate ranking.  An error is thrown if the
            ranking does not exist.

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")