Need to exclude selected values in another filter

[#1]

I have a unique situation with

Country, CountryGroup

A,

B, BB

C, BB

D,

E, CC

I have metrics with Country as the dimension. I would like to display all values for Country excluding those values selected in Country Group. If BB is selected, then countries associated with CC + null() values in Country Group should be displayed.

The metric has 6 nested variables as components which makes it overbearing to simply embed set analysis in each variable only to be recomputed for each selection.

I have tried in expression

1. SUM({<Country = {*} - P({<CountryGroup = {$(= Concat(DISTINCT CountryGroup, ',')) }>} Country)>} variable)

2. SUM({<Country -= P({<CountryGroup = {$(= Concat(DISTINCT CountryGroup, ',')) }}>} Country)>}  variable)

As calculated Dimension

1. If (Match([CountryGroup], CONCAT(Distinct CountryGroup, ',')) = 0, Country, null())

all to no success. Any thoughts? There can be spaces in the Group field so I also tried with chr(39) around the concat with ',' as the delimiter