alternative obligatory attributes

Hi,

is it possible to define alternative obligatory attributes for an element? What I want is something like the following,
where the element otto must have either of two attributes but not both. If it is possible, how would one do this?

<top>
	<file uri="xxxx"/>
	<file relative-path="yyyy"/>
</top>

Thank you for your interest.


Erwin Klöck
   Bayerische Landesbank 
   Geschäftsbereich Financial Office, IT and Operations
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