[Zope] Overriding Z Class methods in instances

This must either be a stupid FAQ, or I'm missing something. It seems you cannot overload the methods of a Z Class in an instance of that class. If you try to, you get a message that the method's id is invalid since it is already in use. If this is really the case, then how can one achieve the necessary separation between presentation and business logic? E.g. if I have a Z Class with an index_html method, all instances of that Z Class will display in exactly the same way. -- Johann