Right now there's no way of saying "if X or Y then do this" nor "if X and Y then do this". Clear usecase is adding vocals/instruments to member of rels, where it should display different things depending on whether there are "vocals or instruments" or "nothing" - but I'm sure there are more.

Luks proposed "

where % is used to get the content of the attribute, for multiple attributes it would simply join the values"