Currently it's not possible to take a multi-valued variable (%performer%, %composer%, %genre% are typical), transform it in tagger script, and then set it back as a multi-valued variable.
Would convert [Electronica, Hip Hop] to "electronica; hip hop" in a tag.
Propose a new function which will unpack a string to a list based on a splitting character; defaulting to the same string.
It's a bit hacky, but it'd be a major piece of work to make the scripting support lists internally (it all assumes text currently, which is very convenient) so this is a reasonable workaround to allow this kind of stuff.
The above example would be done as
Users could also use this to manually set multi-valued variables like