Re: [xml-dev] Namespaces and overrides

[Michael Kay <mike@saxonica.com>]

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iCalendar is a very loose standard in which nearly every component, and every element of those components, is optional. As we revise, extend, “inherit” those components, we often need to make certain of their elements mandatory for some of these service interactions. What are the language / descriptive formats, preferably machine readable, that you would use to tighten specifications in this way as you adopt them for specific re-use scenarios?

XSD tries to do this with complex type restrictions, but I would avoid that route if possible, partly because it involves restating all the things that don't change, partly because you have to define complex type resctrictions at every level of the XML hierarchy. I would rely on XPath assertions to define additional constraints from those in the base schema - expressed either using XSD 1.1, or Schematron, or simply an XSLT stylesheet.

Michael Kay
Saxonica