code: public SingleOutputStreamOperator<T> sum(int positionToSum) {
  		return aggregate(new SumAggregator<>(positionToSum, input.getType(), input.getExecutionConfig()));
  	}
docs: Applies an aggregation that sums every window of the data stream at the
  given position.
  
  @param positionToSum The position in the tuple/array to sum
  @return The transformed DataStream.