How to apply a license based on excluding tags?

Hello Symbiostockers,

Some will still remember me from the early days of Symbiostck. My legacy Symbiostock site (http://www.imagoborealis.com) is still working fine. However, I got some spare time toying with the new Symbiostock plugin, learning woocommerce and preparing to migrate my 4000+ pics.

First tests went well, but now that I have set up a structure of different licenses I am stuck:

I need to apply a license when a certain keyword (tag) is NOT existing in the pic’s meta. In the relevant documentation I find “Apply licenses based on tags (include certain keywords, exclude others)” but I can’t find instructions on exactly how to exclude a certain keyword. Obviously, I tried “-” right in front of that tag but that didn’t work. The license doesn’t get applied to any picture at all (presumably because none of them has a tag of “-keyword”. Any ideas?