Also, if your intention is to create a function which, when passed an argument, returns Q+WOther after replacing WOther with the argument, then the w_ in the left side of the function definition is a pattern which matches any argument, and can be referred to by the name w in the function definition on the right hand side. Just like in the rule in my first response. So w is a named formal argument.

Below I do this and we see it works. But note that when "=" is used to assign the function definition to the function symbol, it causes the right hand side to be evaluated before the asignment is made. That works here, but is often not what is wanted. In the definition of ZZ, the use of ":=" is a deferred assignment. The right hand side is assigned without evaluation. (Therefore no "Out.")  Rather it is evaluated when the function is called, after first replacing the instances of the formal parameter(s) with the associated argument(s).

(Note that I eliminated the subscripting to make the code in the code box more readable. )