How do you return a subclass instance from a base class method?

[Daniel via Digitalmars-d-learn]

I have SubclassOf derived from PosetRelation.  For any poset 
relation, the transitivity law applies, however, I'd like to 
return the correct type:

```
   PosetRelation transitivity(PosetRelation R, PosetRelation S)
   {
      if (R.op == S.op)
      {
         if (R.right is S.left)
            return new SubclassOf(R.left, S.right);
      }
      return null;
   }

```

How does one accomplish this in D?  Because PosetRelation doesn't 
know about SubclassOf, in general.