[Zope] XML model

I am trying to model the following: <!ELEMENT X (A , B), (C, D | E, F) which I can't do as the above as I can only use one kind of connector. I am trying to get the instance to be either: <x><a> <b> <c> <d></x> or <x><a> <b> <e> <f></x> Any / all ideas would be welcome. Thanks Jeremy