Dlang Polymorphism doesn't work as expected.

[Enjoys Math]

I have the following MWE:

```
import std.exception;
import std.stdio;

abstract class B
{
    B opCall() {
       throw new Exception("NotImplemented");
    }
}

class A : B
{
    override B opCall() {
       return new A();
    }
}

int main() {
    B a = new A();
    a();
    readln();
    return 0;
}

```

But B.opCall() gets called!   I thought polymorphism was cool 
until now....

How can I get this to work the way I expected it to?  Would an 
abstract method in base do it?