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See: http://www.w3.org/XML/Group/2007/09/scds We believe that the requirements for SCDs should include a requirement to support metadata querying in an XQuery or XSLT environment. This entails two requirements. First, it is a requirement that SCDs be able to denote arbitrary schema types or element declarations. (We believe the current draft supports this requirement adequately.) Second, in the long run a full-scale search facility on schema components will be needed, to enable finding things like "the set of types derived from type T" or "the set of elements substitutable for element E". Some members of the QT Working Groups expressed reservations on this topic, and whether search facilities should be embedded in SCDs or supplied by an environment like XQuery.
The WG considered this comment at the October 2007 F2F and agreed to add a requirement related to this, but limiting the scope somewhat from the full query language implied by part 2 of this comment. We will also explore reverse axes and related mechanisms.
Fixed in Last Call draft http://www.w3.org/TR/2008/WD-xmlschema-ref-20081117/