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During the review, we discussed whether we prefer one or the other. It quickly showed, that there are use cases and needs for both. E.g., complex type derivation is easier to map with popular type to object model mappers while substitution provides for better XPath discoverability of the information. We propose (after talking to one of the editors) that the spec should provide both approaches with some guidelines on when one may be preferable over the other and provide a transform between them (as it was available in previous internal drafts)
The Query WG has considered your comment and has determined that two approaches are unnecessary and undesirable, but agrees that a substitution group approach is preferred. A new XQueryX schema has been developed that replaces the former complex type derivation approach with a substitution group approach. When a new edition of the XQueryX spec is published, we hope that you will review it and, if, you find the changes satisfactory, mark this comment as CLOSED. We appreciate your feedback on the XML Query specifications. Please let us know if this response is satisfactory. If not, please respond to this message, explaining your concerns. Jim Melton On behalf of the XML Query Working Group