I am talking rubbish again. Sorry. %_multiartist% is set to 1 when there are more than one primary artists across all of the tracks in the album. So if you ignore my previous post here is a corrected version:

The change for _multiartist is nothing to do with NAT, and you would probably not want to use %_multiartist% as the basis for unsetting %albumartist%. For example, [10cc's Greatest Hits… and More](http://musicbrainz.org/release/83240d6a-692f-4ebc-981c-5a4147093fa7) has tracks by Godley & Creme, Wax etc. so %_multiartist% is 1 yet this is definitely an album officially attributed to 10cc so you would not want to unset the albumartist. 

The `$if($ne(%albumartist%,),%album%/)` is designed to complement the existing `$if2(%albumartist%,%artist%)` i.e. if we are using %artist% for the top-level directory rather than %albumartist% then it is probably a NAT track, and that is why I have used the same test (but expressed differently) for deciding whether to use the next directory down.

NOTE: I just realised another issue here - which is that %artist% could be e.g. "Elton John feat. Kiki Dee" - and for a NAT track we would probably want something like `$if2(%albumartist%,%artists[0]%)` i.e. the primary artist here but AFAIK there is no easy way of getting that reliably from a script variable at present. So, perhaps I should add a $index(multi-value, x) script function (x starts at 1 rather than the python 0) and use $if2(%albumartist%,$index(%artists%,1)) for the top-level directory.