type Holder {
    value: ref<i32>,
    fn replace(self: mutref<Self>, next: ref<i32>) -> Void {
        self.value = next
    }
}

fn choose(flag: Bool, left: mutref<Holder>, right: mutref<Holder>) -> mutref<Holder> {
    if flag {
        return left
    }
    return right
}

pub fn main(flag: Bool) -> Void {
    var x: i32 = 1
    let y: i32 = 2
    let z: i32 = 3
    var left: Holder = Holder { value: &x }
    var right: Holder = Holder { value: &y }
    let selected: mutref<Holder> = choose(flag, &mut left, &mut right)
    selected.replace(&z)
    x = 4
}