content model question

Hi all,

Due to the non-deterministic nature of XML Schema, I was wondering if 
it  is possible at all to define this model:

Allowed:

<a>
   <b/>
</a>

<a>
   <c/>
</a>

<a>
   <b/><c/>
</a>

<a>
   <c/><b/>
</a>

Not allowed:

<a/>

Does anyone has an idea?

Regards,

</Steven>
-- 
Steven Noels                            http://outerthought.org/
Outerthought - Open Source, Java & XML Competence Support Center
stevenn@outerthought.org                      stevenn@apache.org