Accessing grouped values "inside" fields

Hello,
I'm struggling to make operations on results obtained from grouping by
specific dimensions. To be more specific, I'd like to substract value of
e.g. sales for some product for one year from values from the preceding
year.
I'd like to somehow access the resulting recordset from grouping and make
some operations on these values, just like the SUBTOTAL function does.
Thanks in advance for any help.
Best wishes,
Marek T³uczekOne correction to my question:
> I'd like to somehow access the resulting recordset from grouping and make
> some operations on these values, just like the SUBTOTAL function does.
I meant not recordset, but set of fields.
> Thanks in advance for any help.
> Best wishes,
> Marek T³uczek
>
>