What is the issue with  in rainbow? ... Do you have an example?
I think you'll find it's a very simple example. :-p
rainbow.parser.ParseException: Encountered "]" at line 1, column 2.
As for 'macex1, there's definitely one inconsistency, which is that it doesn't work unless car.expr works. I also briefly worried about ssyntax, but I'm not sure it's a problem; official Arc lets (assign a.b nil) cause (bound 'a.b) to be true, but I don't blame Rainbow for not emulating that.