choice for attributes?

hi there,

         How do I validate an element {If} which must have exactly ONE 
attribute out of a set of two attributes {defined,notDefined}:

<If defined="..."> 	OR <If notDefined=".."> (both valid)
BUT NOT <If defined="..." notDefined="..."> AND NOT <If> (both not valid)

I would need anything like <xs:choice>, but this does only exist for 
elements, not for attributes.


Many thanks in advance,

Urs.

Urs Muntwyler
A&F Computersysteme AG, Switzerland
umuntwyler@a-f.ch