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In section 3.5, the draft spec says If an SML identity constraint is specified for an element declaration E, then this constraint is applicable to all instances of E in a model, i.e., the identity constraint MUST be satisfied for each instance of E in a valid model XSD 1.0 does not impose any analogous constraint on identity constraints within single documents. Should we align with XSD by removing this constraint? Should we align with XSD by persuading the XML Schema Working Group to impose an analogous constraint in XSD? Or should we decide that on this point we believe alignment with XSD is not desirable or necessary? Some thinking and research will be needed, and discussion with the Schema WG.
The quotation seems to be out of synch with the summary of the issue. On the call of 2 August, the WG concluded that the passage we had intended to point to was a different bullet item in what is now section 3.4 on Identity Constraints. Each SML identity constraint that is specified for a global element declaration G MUST be treated as if it is specified by default for all global element declarations SG that are in the substitution group hierarchy whose head is G
Constraints are an integral part of a type definition or element declaration in SML and can not be removed/overridden by derived types or element declarations in a substitution group. We should NOT align with XSD 1.0 here, but we can consider making a request to the schema working group to consider aligning XSD 1.1 with SML.