code: def sum(self, attribute_selector, state_keeper=None):
          
          op = Operator(
              _generate_uuid(),
              OpType.Sum,
              "Sum",
              _sum,
              other=attribute_selector,
              state_actor=state_keeper,
              num_instances=self.env.config.parallelism)
          return self.__register(op)
docs: Applies a rolling sum operator to the stream.
  
          Attributes:
               sum_attribute_index (int): The index of the attribute to sum
               (assuming tuple records).