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   derivation by restriction and the use attribute

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I am currently experimenting with derivation by restriction and attribute declarations. I tried all combinations of changing the attribute use="required|prohibited|optional" from the base type to the derivation. The following two combinations of are intuitively valid.
(1)Basetype: use="optional" Subtype:use="prohibited"
(2)Basetype: use="optional" Subtype:use="required"
These combinations seem to be a clear restriction of an attributes cardinality from (0,1) to (0,0) in case 1 and to (1,1) in case 2.
As the XML Schema specification is quite complex I rely on XSV and IBMs Schema Quality Checker to find out the valid combinations.
XSV doesn't bother about changing the attribute through derivation by restriction - all combinations are valid.
SQC grants three combinations: case 1 and 2 and a third one:
(3) Basetype: use="required" Subtype:use="prohibited"
This case is not restriction. (In my understanding of a restriction)
So my question concerns the validity of this third case. Is this case valid? Where is it specified in the Schema Specification?
Helmut Dirtinger


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