It looks like you are attempting to modify the parameter that is passed in to the function itself, by using a style similar to what other languages would have if you could pass params via reference. A reference is simply an address that points to a different variable. So in languages that support stuff like this, you can modify the original parameter that is passed in directly.
I do not believe Verse supports this, and instead uses pass by copy. Therefore you just need to manipulate the parameter however you want using another temporary variable to store the result, and then return that temporary variable.