[Zope] [Q] More baseclass, subclass, nested class confusion ...

[Darran Edmundson]

I'm still struggling with through-the-web ZClass definitions.
The test problem I am using as an example is the Actor-Participant
object model proposed by Phillip Eby (thanks Phillip!).  The
people at my academic site each have a folderish PersonClass.  
Academics typically proceed through undergrad, graduate school,
postdoc or alumni, etc.  So, beneath the PersonClass I have created
an ObjectManager-based PositionClass with properties StartDate
and EndDate.  I want to use this as a base class for further
refinements: DegreeClass, PostDocClass, ProfessorClass, 
SabbaticalClass, ie.,

PersonProduct
    - PersonClass (ObjectManager)
          PersonClassProperties (FirstName, LastName, Email, URL)
        - PositionClass (ObjectManager)
              PositionProperties (StartDate, EndDate)
        - DegreeClass (PositionClass)
              DegreeProperties (School, Degree)
        - AlumniClass (PositionClass)
              AlumniProperties (CurrentEmployment)
        ...

The idea is for individuals to build up a chronology of
positions by adding instances of these PositionClass-derived 
ZClasses to their respective folders.  Here PositionClass is 
meant to be an abstract class.  The user should not be able 
to create an instance of this class but I cannot figure out
how to prevent PositionClass from being an "available object"
under PersonClass instances.  Any help is much appreciated.

Darran.