code: def reduce(self, reduce_fn):
          
          op = Operator(
              _generate_uuid(),
              OpType.Reduce,
              "Sum",
              reduce_fn,
              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).