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The content model for xs:schema is given as Content: ((include | import | redefine | override | annotation)*, (defaultOpenContent, annotation*)?, (((simpleType | complexType | group | attributeGroup) | element | attribute | notation), annotation*)*) As far as I can tell the parens around (simpleType | complexType | group | attributeGroup) are completely redundant. (It's a shame that the annotation elements in this content model can't be described using openContent - it would be possible if a wildcard could define a list of permitted QNames, rather than just a list of forbidden QNames).
Covered by the proposal adopted by the WG on 2008-09-26.