Real4 plus

Maybe, we should even use (-foo).is_positive for the second helper (\_eval_is_negative)?
I would prefer to not delay. If the sign is unknown we could exit right now so I would like to still have the mutable loop (which, last time I checked, was more efficient that I thought) and do:

```
s = S.ImaginaryUnit*C.sign(C.im(t))
if type(s) is int:
    sign = s*sign
else:
    args.insert(0, s)
```