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   [schema] which assertions can't you express ?

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An XML Schema is the expression of some assertions expected on an XML 
document class. Assertions on XML documents ensure that applications 
will process them without causing faults. Expressing assertions with 
schemas ensure that applications developpers will spend most of their 
time in designing data process and few of their time in controlling them.

Any schema technology is designed to cover numbers of assertions 
expressed. However, if think that the existing schema technologies (DTD, 
W3C XML Schema, Relax NG, Schematron) can't express some constraint types.

I have some ideas of what these constraint types might be, but I'd like 
to know, helped by your own experiences, which kind of useful assertions 
can't be expressed with these schema technologies (so frustrating).

For example :
-i'd like that the number of occurrences of an element is equal to a 
value of a given attribute (Schematron could do that)
-i'd like that the value of a given attribute exists in my RDBMS

Thanks in advance

           (. .)
|   Philippe Poulard    |


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