I'm guessing this would be most useful when interfacing with naughty code that you can't rewrite. E.g. you need to call a function from another library that doe

I'm guessing this would be most useful when interfacing with naughty code that you can't rewrite. E.g. you need to call a function from another library that does something useful and also modifies its argument, and you only want it to do the useful thing.