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This issue was originally reported by Paul. V. Biron. The suggestion is straightforward: Allow an element declaration to be in more than one substitution group. See (member-only link) http://lists.w3.org/Archives/Member/w3c-xml-schema-wg/2002Sep/0016.html (http://lists.w3.org/Archives/Member/w3c-xml-schema-wg/2002Sep/0016.html).
I have slightly mixed feelings about this proposed change (allowing elements to belong to multiple substitution groups). I think the restriction to a single substitution group is unmotivated and causes little but pain to users of the spec. In the use cases I have in mind, allowing things to be in multiple substitution groups will however tend only to make it easier to fall afoul of the determinism rules, so the net gain for users is not as great as one would like. On the other hand, the demand for this change remains very modest, and given the choice between deciding not to make this change in 1.1 and delaying 1.1 further, I am inclined to believe it's better to say "maybe later, but not in 1.1" and close this issue. So I propose we close this as RESOLVED / LATER. Sorry, Paul.
The working group discussed this briefly at its telcon today; we declined a proposal to close it as RESOLVED / LATER.
The working group accepted a proposal to allow elements to have multiple substitution-group heads at our New Orleans face to face meeting 31 January - 2 February 2007. Final amendments to the wording adopted then were approved on 23 February. Accordingly, I'm marking this issue RESOLVED. The originator of the issue, Paul Biron, is a member of the Working Group and thus presumed to be aware of the resolution of the issue; he registered no objection. So as soon as it's feasible, I'll change the status of this issue from RESOLVED to CLOSED on his behalf.