How can I check a newly set ref value

[solidstate1991 via Digitalmars-d-learn]

Let's say I have something like this:

```
struct Foo {
    int[] bar;
    this() {
        bar.length = 10;
    }
    ref int opIndex(size_t i) {
        return bar[i];
    }
}
void main() {
    Foo f = Foo();
    f[3] = 15;
}
```
If I wanted to check whether the assigned value is within a 
range, and I want to throw a certain exception if outside of it, 
then how I can do that?