cannot I override and mark it as private?

[Jack via Digitalmars-d-learn]

I'd to change the visibility of a method overrided from public to 
private but it doesn't work tho to protected it does. Why is that?

give:
```d
class A
{
    void f() { }
}
```

this is ok:

```d
class B : A
{
    protected override void f() { }
}
```

this is not:

```d
class B : A
{
    private override void f() { }
}
```

Why is that? why must I leave it accessible somehow (even if it's 
protected) to all derived class of my derived class?