How to access private variable of outer class from an inner struct

[thebluepandabear via Digitalmars-d-learn]

If I have the following code:

```D
class X {
    private int num;
    struct Y {
        // how to access num?
    }
}
```

How would I access `num` from `Y`?

Whenever I try to I get a compilation error.

I believe that it's possible for nested/inner classes, but I 
don't know if it's possible for structs.

Help would be apprciated.