code: def pick(self, keys):
          '''
          Returns an instance with just those keys
          '''
          subset = dict([(key, self.criteria[key]) for key in keys])
          return self.__class__(**subsetf pick(self, keys):
          '''
          Returns an instance with just those keys
          '''
          subset = dict([(key, self.criteria[key]) for key in keys])
          return self.__class__(**subset)
docs: Returns an instance with just those keys