>I do not believe that you can model a framework in advance.

Of course you can model it, but that doesn't mean you'll never change the model (or, more frequently, its implementation).

> if you framework does not work right, and you do not know that in advance, you are in trouble, because you cannot change parents.<

I'm not sure what you mean, here. Certainly, changing interface can mean cascading changes to the app code, and that's a pain. You really do want to make as sure as possible that your public methods (and the parameters they take) stay the same. But beyond that, you're free to change the implementation as much as you want. One of the nicest things in the world is to fix a bug in the framework and have that fix automatically cascade to your apps.