docs: public SortedGrouping<T> sortGroup(int field, Order order) {
  		if (this.getKeys() instanceof Keys.SelectorFunctionKeys) {
  			throw new InvalidProgramException("KeySelector grouping keys and field index group-sorting keys cannot be used together.");
  		}
  
  		SortedGrouping<T> sg = new SortedGrouping<T>(this.inputDataSet, this.keys, field, order);
  		sg.customPartitioner = getCustomPartitioner();
  		return sg;
  	}