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The {attributes} property on the Annotation schema component is described as "A sequence of attribute information items, namely those allowed by the attribute wildcard in the type definition for the <annotation> item itself or for the enclosing items which correspond to the component within which the annotation component is located." http://www.w3.org/XML/Group/2002/09/xmlschema-1/structures-with- errata.html#cAnnotations http://www.w3.org/XML/Group/2002/09/xmlschema-1/structures-with- errata.html#declare-annotation The XML Information Set spec, on the other hand, defines the [attributes] property of element information items as containing a set, not a sequence, of attribute information items. http://www.w3.org/TR/xml-infoset/#infoitem.element I think the XML Schema spec should probably similarly specify a set, not a sequence, of attribute information items. See: http://lists.w3.org/Archives/Public/www-xml-schema-comments/2003JanMar/0089.html
A proposal to resolve this issue has been placed at http://www.w3.org/XML/Group/2004/06/xmlschema-1/structures.bannotations.200610.html (member-only links) for review and action by the WG.
IIRC the reason for switching to a sequence for {attributes} was so that a (partial) ordering could be relied on, i.e. if an annotation A derives from a higher element than an annotation B, it would precede that annotation in {attributes}
The proposal mentioned in comment #1 was accepted by the WG during its meeting of 20 October 2006. The changes have been integrated into the status quo document and will appear in due course in the next published working draft.