[Set analysis] subtract set using As Of Month rolling periods

[cicciput]

Following community advice I implemented an AsOfMonth table in my load: Calculating rolling n-period totals, averages or other aggregations

So I have such data:

and now I can count:

• how many buyers in current month Sum({<As_Of_Month.Type={'Current'}>} Account.DCF)
• how many buyers in rolling quarter Sum({<As_Of_Month.Type={'Rolling 3'}>} Account.DCF)
• how many buyers in previous rolling quarter Sum({<As_Of_Month.Type={'Prev. rolling 3'}>} Account.DCF)

Now my question: how do I count how many buyers purchased in previous rolling quarter but not in the next rolling quarter?

I assumed this but it does not work:

Sum({<As_Of_Month.Type = {'Prev. rolling 3'} - {'Rolling 3'} >} Account.DCF)

as it counts the same as Sum({<As_Of_Month.Type={'Prev. rolling 3'}>} Account.DCF)

Any advice welcome.

[cicciput]

Is my request something you simply can't get from Qlikview?