Filtering documents without tags
I've been using Zotero as a main support tool for the writing of my master thesis, however there's something I haven't been able to do: I tag every document I read, not only to categorize them, but as a way to mark that I've actually read them. The problem with this approach is that I haven't found a way to filter or find all documents which have no tags attached (which would be my remaining "to do list"). Is there a way to do so? I've also searched the extensions and couldn't find anything that would suit this need. Any tips?