[plt-scheme] OOP problem - cloning objects

Hi,

I've stumbled over an oop problem that probably has an easy standard 
solution but right now I'm confused. I have several subclasses of 
MrEd's text% class, like e.g. text% > z-text% > z-entry%.

Now I need to create a deep copy of instances of such classes. 
Therefore, I have created an interface clonable-object<%> with a method 
clone that is supposed to return a deep copy of the resp. object.

Problem: Each superclass sure knows how to clone itself, but naturally 
it can only return an instance of its own class in its clone method.  
It would seem natural for z-entry% to rename the clone method of 
z-text%, calling the superclass method, cloning any  information that 
is new to z-entry%, and return a new instance of z-entry%. But the 
superclasses clone method will return an instance of z-text%.

Does that mean that e.g. z-entry% has to re-implement all of the clone 
code that is already in its superclass z-text%? Is there a simple trick 
I've overseen to clone objects such that each derived class only 
provides "its own part" to the cloning process? Is there some way to 
cast an object into an object of a derived class?

Regards,

Erich