Redefinition
Simple content models can be extended by means of a union, similar to a union in C or a Redefine in COBOL. I haven't seen many applications for it. A similar-sounding but fundamentally different feature is the schema xs:redefine Element. The xs:redefine Element can be used to modify the definitions of an external schema before they are used within the redefining schema. Some people may find reasons for doing this, but the extension and restriction mechanisms work just as well in most cases.
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