I'd guess senior level candidates should solve it without copying and pasting, perhaps make sure test cases are present for the behaviour before modifications a

I'd guess senior level candidates should solve it without copying and pasting, perhaps make sure test cases are present for the behaviour before modifications are made, and then generalize the existing functions. Or, state why adding more functionality on the server side for every type of arithmetic operation would be unnecessary complexity, and instead add support for any type of atomic arithmetic operation with a transaction/UDF approach.