Sorry for your trouble, as many discusses, FBC compiler treat (5)+ShowNum(10) as first ShowNum parameters, just like ShowNum((5)+ShowNum(10)).
I'm not a compiler expert, but if the token after the first ShowNum is a left paren, then its parameters should be 5( the things between left paren and right paren ), not (5)+ShowNum(10)
I think it as angros47's test, it can be compiled pass that the FBC need treat (5)+ShowNum(10) as parameters, or it shows error 10: Expected '=', found 'ShowNum' in 'ShowNum+ShowNum', but in my thoughts, show the error message is more reasonable.