Specific questions about working with variables

How do I work with a variable from the “Last FMng” plugin? According to the “View Script Variables” plugin, the field is named “comment:Songs-DB_Occasion.” When I try to plug that into the tagging script I get a message about an unexpected character “-”

How do I set a tag to “Null,” or remove the tag entirely through the script? I have some specific tags I’d like to build some logic around. My current intent is to use the “Lyrics” field to add “Instrumental” to “Genre” if it is equal to “Instrumental,” and remove it altogether if it is not. I do have some other potential use cases in mind, but I need this piece first.

*Edit to fix a typo.