complex uniqueness

Hi,

Does anyone knows if its possible to create a xs:key to guarantee that the below instance document could have a uniqueness constraint among names A,B,C,D of only the 2nd,3rd and 4th pointSets (they are linked via the attribute 'extends')

Carlos

<root>
	<pointSet type="ordinary">
		<point name="A"/>
		<point name="B"/>
	</pointSet>
	<pointSet type="basic">
		<point name="A"/>
		<point name="B"/>
	</pointSet>
	<pointSet type="complex" extends="basic">
		<point name="C"/>
	</pointSet>
	<pointSet type="verycomplex" extends="complex">
		<point name="D"/>
	</pointSet>
</root>

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