Applies a rolling sum operator to the stream.

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

def sum(self, attribute_selector, state_keeper=None):
        """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.Sum,
            "Sum",
            _sum,
            other=attribute_selector,
            state_actor=state_keeper,
            num_instances=self.env.config.parallelism)
        return self.__register(op)