[Zope] ZClass in 2.8.4 - error help

[Jake]

I am trying to get an old, old product to work in 2.8.4 and I am  
getting the following error:

Traceback (innermost last):
   Module ZPublisher.Publish, line 113, in publish
   Module ZPublisher.mapply, line 88, in mapply
   Module ZPublisher.Publish, line 40, in call_object
   Module Shared.DC.Scripts.Bindings, line 311, in __call__
   Module Shared.DC.Scripts.Bindings, line 348, in _bindAndExec
   Module App.special_dtml, line 175, in _exec
   Module DocumentTemplate.DT_Let, line 76, in render
   Module DocumentTemplate.DT_In, line 643, in renderwob
   Module DocumentTemplate.DT_In, line 771, in sort_sequence
   Module ZODB.Connection, line 704, in setstate
   Module ZODB.Connection, line 760, in _setstate
   Module ZODB.serialize, line 495, in setGhostState
   Module ZODB.serialize, line 488, in getState
   Module ZODB.serialize, line 399, in _persistent_load
   Module ZODB.serialize, line 372, in _get_class
   Module Zope2.App.ClassFactory, line 29, in ClassFactory
   Module OFS.Uninstalled, line 61, in Broken
   File "<string>", line 1

     class <class 'ZDTopic_ZPropertySheetsClass'>(BrokenClass): ' ';  
__module__=None

           ^

SyntaxError: invalid syntax

I know, that it has something to do with the new extension class, but  
I can't seem to find any documentation on the new classes, how to  
write them or how to convert old classes?

Any pointers?

Jake

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"Zoping for the rest of us"