Use nuniqueval attribclass "pointgroup"

[DBaciu]

Does anybody know how to use a group name as attribute class (attribclass) in a function like nuniqueval????

In the docs it says it is possible: 

attribclass

One of "detail" (or "global"), "point", "prim", or "vertex".

You can also use "primgroup", "pointgroup" or "vertexgroup" to read from groups.

 

I can't figure it out how to use this thing, it doesn't work. Basically I want to find the number of unique attrib values from a group of points, not from all points in the geometry. For example I did something like this (point groups with the name of the primitive are created earlier)

string grp = sprintf("primnum_%g", @primnum);
int islands = nuniqueval(0, grp, "island");

i@islands = islands;