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Don Box: The specific epiphany I had today was as follows: There is a sweet spot in XML Schema that combines element wildcards (e.g., <xsd:any>) and D-B-R [derivation by restriction]. Specifically, if one terminates the content model of a complex type with the traditional element wildcard:
<xs:any minOccurs="0" maxOccurs="unbounded" />
That one can easily emulate D-B-E [derivation by extension] simply by peeling N particles in the derived type, one for each "extended" element.