The symbols "!" and "?" stands for addition and subtraction, (not necessarily in that order) but for subtraction it can be the left argument subtract for the right or vice versa. e.g. a!b can be a-b, a+b or b-a. Variables and brackets can be used as usual. How to write $20a-18b$ for sure, with 'a' and 'b'?
My attempt: in the contest I found an expression that equals to 2(a-b), but I can't remember what it was. It's like ((a!b)?a)!((b?a)!b) or something like that. But I don't know how to add two different things in general.
Is it possible to form an expression that represent a+b?