code: def add_mismatch(self, entity, *traits):
          
          for trait in traits:
              self.index[trait].add(entity)
docs: Add a mismatching entity to the index.
  
          We do this by simply adding the mismatch to the index.
  
          :param collections.Hashable entity: an object to be mismatching the values of `traits_indexed_by`
          :param list traits: a list of hashable traits to index the entity with