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   RE: [xml-dev] Are people really using Identity constraints specifie

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And this is the crux of the problem here as you describe it:

XML Schemas is admittedly not powerful enough to express contingent
constraints well--they are often dynamic--, but it is rather inconvenient
for expressing static necessary constraints. In part this is because of
content models: you really have to fight with grammars to make them open
and extensible. In particular, many people are aware that enhancing a
derived schema often involves rejigging the base schema. It is because XML
Schemas started from the wrong position: "lets have base schemas and be
able to derive schemas from them" rather than "lets have necessary schemas
and be able to declare extra contingent constraints", which would have
made "open" content models the default, for example. (This is an issue
Roger championed, unfortunately too late!)
Quoting rjelliffe@allette.com.au:

Both Schematron and CAM take the opposite approach and allow the baseline
structure and extra varients to be declared dynamically.

I feel at times like the W3C Schema TC is continuing to attempt to squeeze the
last few ounces out of their 'paddlewheel sternwheeler' design - while we now
have screw propellers available...



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