Something like object! -> _CHECKED("object", object) might work.
Hrm. The object! -> _CHECKED( "object", object) approach is awkward for a variety of reasons.
So I'm still probably trying to be too clever here. A "tested index" semantic would be straightforward enough if I gave up on making it compatible with Peter Shook's table unpack syntax. The best approach is probably just to make an operator that handles the field selection case exclusively. I.e., one who's only purpose is to error properly on 'table!.key'. Then the tested table unpack use case could be handled with an independent operator, like 'vars in! table'.
-Sven
