Re: xml attributes

Regarding the question of validating choices with xsd:attributes (i.e. that 
using one attribute should preclude the use of the other) I ask why is it 
that the schema for schemas itself has this requirement (for example the 
"attribute" element can have either the attribute "name" or "ref" but NOT 
both), and yet it is not backed up by XML Schemas 1.0?

regards
David

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