There kind of is negative and positive side to an axis - in fact, all inputs are mathematical expressions, so to add a negative side to both you would just do:
aimx=D-A
aimy=S-W
You can also use parenthesis `()` and basic operators `+ - * / % < <= > >= == <> && ||`, so it might be possible to finetune keyboard aiming to a usable state. If not, I'll be looking to further expand what is possible to do with this in future.