is it possible to set the attributes for semi additive mesures with an rule at the attribute Cube?

eg:

['#_Periode_':'TimeAggregation'] = if(1 == 1, "Special2", "")

in my case this rule is not working -- what is wrong?

solved:

['#_Periode_':'TimeAggregation'] = B:if(PALO.DATA("","Leistung",!'Periode',"ZG","Leistung","ZGEUR","Periode","Ist") == null, "Special2", "Special1")

But now all Parent Elements also get the "Special1" attribute if it is set in one Month below. How can i stop this?