code: def top_expression_effect(self, expression_levels):
          effect_expression_dict = self.effect_expression(expression_levels)
          if len(effect_expression_dict) == 0:
              return None
          def key_fn(effect_fpkm_pair):
              (effect, fpkm) = effect_fpkm_pair
              return (fpkm, multi_gene_effect_sort_key(effect))
          return max(effect_expression_dict.items(), key=key_fn)[0]