Checking for DLC in SmileBasic 3

I want to know if a DLC is owned by a certain individual once they download my program. How do I do this?

EXTFEATURE is true when the DLC is present, so you can write stuff like:

IF EXTFEATURE THEN
 ?"You have the DLC!"
ELSE
 ?"You don't have the DLC!"
ENDIF

Oh good, another thing totally missing from the reference. Thank you!

The Japanese version of the reference is more complete, though of course it's not in English. There's also the in-game help, which does include EXTFEATURE, but it's harder to find something if you don't know what it's called.