(no c) indeed does what I want, but I find its name counter-intuitive and not psychologically sound. Saying (if (no x)) does not sound as natural as (if (not x). I could define not by saying
(= not no)
Or:
(def (not x) (no x))
Etc. I recall searching for not in the arc sources and getting nothing because "no" was used instead. It's "not" in Perl 5, Ruby, etc. - not "no", so I assumed Graham would use it, but he didn't.
Regarding the tests - I can integrate them into my own test suite, but I only accept code that is under the MIT X11 licence, so I may not be able to copy-paste code like that. (I still don't know what licence the original Arc code is under).
Regarding your suggestion for implementation/version declaration, this seems interesting and is supported in TAP using TODO and SKIP tests, but will naturally require more logic and complexity.