Applies a rolling sum operator to the stream.

        Attributes:
             sum_attribute_index (int): The index of the attribute to sum
             (assuming tuple records).

def reduce(self, reduce_fn):
        """Applies a rolling sum operator to the stream.

        Attributes:
             sum_attribute_index (int): The index of the attribute to sum
             (assuming tuple records).
        """
        op = Operator(
            _generate_uuid(),
            OpType.Reduce,
            "Sum",
            reduce_fn,
            num_instances=self.env.config.parallelism)
        return self.__register(op)