Subtracting group footer sums

[mirce.vladimirov]

I use group header in my reports, so i also have group footer which presents the sums of items contained in the master detail of a single group.
In the bottom of the report, i have a band "Report Summary" whic should calculate and present the  total on the whole report. Calculating a sum of values goes very simple : in the given memo's editor i write some code like :

Code: [Select]

[SUM([myLazarusQuery.MyFloatfield],BandName)]
But there is a case when I should subtract some groups from the total of all sums.
There should be a way to subtract totals of a certain Group, since there is a Grouping criteria-field, how can I do that ?