This is going in here because it could either be a bug, an undocumented feature, I don't know what this is.
Mariominer (Zee on Miiverse) posted this thread, stating he found a functional form of USE that returns an empty string. I was curious, so I tested.
Turns out, it returns nothing at all! It acts the same way as USE with the exception that you write it as a function. You can even leave it alone on its own line and it works (but no OUT form exists.) Tried it on EXEC, it's the same way! Even PRINT, except that it can only accept one expression (no ; or , for you.) This hearkens to the functional form of VAR, which isn't so much a function as it is an alternate way to access a variable. Try all this and it works exactly as expected.
PRINT(20) USE(1) EXEC(1)All of these instructions are marked as keywords in the syntax highlighting. So this leads me to believe that all keywords can be called like such, and may even have alternate features. Needs investigation.