How to make simple type choice

I need to make a choice between various simple types for an attribute.
 The xml may look something like this:

<project>
<queries name="test1">
  <query name="somename1"></query>
  <query name="samename2"></query>
</queries>
<queries name="test2">
  <query name="somename2"></query>
  <query name="samename3"></query>
</queries>
</project>

So basically a queries node named "test1" will have a query node with
an enumeration restriction of:
somename1
somename2

And, a queries node named 'test2" will have a query node with an
enumeration restriction of:
somename2
somename3

How do you achieve this?  I'm looking at xs:assert but not sure I'm on
the right path.
Thanks,
Karl..