reimplementing an interface in a derived class

[kdevel]

https://dlang.org/spec/interface.html #11 has this code example:

```
interface D
{
     int foo();
}

class A : D
{
     int foo() { return 1; }
}

class B : A, D
{
     override int foo() { return 2; }
}

...

B b = new B();
b.foo();            // returns 2
D d = cast(D) b;
d.foo();            // returns 2
A a = cast(A) b;
D d2 = cast(D) a;
d2.foo();           // returns 2, even though it is A's D, not 
B's D
```

What is the meaning of the ", D"? It does not seem to make a 
difference if it is omitted.