Specifying the aggregation of results by subsets of items and their labels

Form: # # # ... : label | # # # ... : label |...

Example

If we have the following list of items:

1- Butane
2- Petrol
3- Gas

4- Cub Sugar
5- Granulated Sugar
6- Powdered Sugar

7- National Flour
8- Free Flour

You may want to aggregate the result for sugar and flour. This may be done by adding the option:
aggr( 4 5 6 : "Sugar" | 7 8 : "Flour" )