Real4 plus

Maybe, we should even use (-foo).is_positive for the second helper (\_eval_is_negative)?
I don't believe you need the processing in `is_positive` since the inference system can figure it out from `is_negative`. That's what I do and don't like about the inference engine. It's not clear when the added code will make a test resolvable. If you think it's needed or preferable then the comment I made below applies here about the mutable args.