I don't see what your argument is against setf being =, Except to maintain some compatibility with CL, which PG already said was not his intent with Arc.
I agree with him. The symbol = means "equality", not "definition". This convension is a bad thing the languages have learned from C (or any older one). The = could be a function to test equality of numbers (maybe for speed improvement) or even instead of "is". The name "is" remembers type checking, not comparison (in my opinion, but this is no the real problem, though).
If briefness is the target, we should use := (or $) instead.
