Like you said, it's {if not seen. It works exactly the same way as seen, just reversed.
If you're having a problem with this, maybe you could provide more information.
I always open a new game to test things out. Can't hurt anything that way.
You can even open a cute, little Squiffy in Scratchpad via this very site's documentation section:
https://docs.textadventures.co.uk/squiffy/scratchpad/
It is an older version of Squiffy, but most things that aren't written by you in Javascript work about the same.
You can also save a Squiffy story file as a GitHub Gist and load the Gist directly in that scratchpad.
Want to edit some stupid game I just saved as a Gist?
https://docs.textadventures.co.uk/squiffy/scratchpad/?gistid=02338c1669580fac501207ddba62c98e&filename=Get%20the%20Gist.squiffy
Want to load your own Gist?
Go to that first link and click the "Load Gist" button.
I learn most things by trial and error.
