conditional preclude

[Phrll]

I'm working on adding the Traits and Gadgets from the Doctor Who sourcebooks to a Doctor Who dataset, using the editor to create .user files for each sourcebook and I've run into a problem that I haven't figured out how to handle yet. Some of the Traits are in more than one sourcebook; for example Arrogant is in the 4th and 7th Doctor books. I added them to both files (different IDs) and both show up in the Chooser but since they're the same, I'd like to set up something so only one shows up even if both sources are being used. I added a Preclude to the 7th Doctor .user file to exclude the 4th Doctor Trait and it works fine as long as both files are present but if the 4th Doctor file is missing, I get "Preclude - Reference to non-existent thing "ab4dArrog". Is there a way to do a conditional preclude that checks for a source?

Thanks!