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Section 3.8.2 permits minOccurs=0 on an "all" element, but 3.7.2 and 3.8.6 (Schema Component Constraint: All Group Limited) prohibit the value from being zero for all uses. See http://lists.w3.org/Archives/Public/www-xml-schema-comments/2001AprJun/0198.html
It was the intention that minOccurs=0 should be permitted on an "all" element. Erratum item E1-2 has been created in the Errata doc.