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The constraint "Content type restricts" reads in part: 1 If BCT's {variety} is empty, then either there is no sequence of element information items which is ·locally valid· with respect to RCT's {particle}, or only the empty sequence is. This seems to assume that RCT has a particle, which is not true if RCT has {variety} of 'empty' or 'simple'. The only part of the spec that depends on this constraint is Schema Component Constraint: Derivation Valid (Restriction, Complex), and the thicket of conditionals there is too thick for me to tell whether any actual harm results from leaving "Content type restricts" undefined for various cases. I think the 'Content type restricts' rule would be more useful if it provided a predicate which is true or false for any pair of content types, instead of providing one which is true for some, false for some, and undefined for some. Making this definition more general might also make possible a simplification of "Derivation Valid (Restriction, Complex)".
A wording proposal for this issue (among others) was sent to the XML Schema WG on 4 February 2008. http://www.w3.org/XML/Group/2004/06/xmlschema-1/structures.consent.200802.html (member-only link) For some issues, the proposal is effectively to make no change; see the Status section of the proposal for the specifics.
During its telcon today, the XML Schema WG accepted the 'Structures Omnibus 2' proposal, which includes changes intended to resolve this issue. (Or, for some issues, contains the editors' proposal that the issue should be closed without further changes.) http://www.w3.org/XML/Group/2004/06/xmlschema-1/structures.consent.200801.html (member-only link) Accordingly, I'm marking the issue resolved. The originator of this issue (or in some cases the individual, acting on behalf of a group, who filed the comment) should receive an email notification of this change. Please examine the changes and let us know if you agree with this resolution of your issue, by adding a comment to the issue record and changing the Status of the issue to Closed. Or, if you do not agree with this resolution, please add a comment explaining why. If you wish to appeal the WG's decision to the Director, then also change the Status of the record to Reopened. If you wish to record your dissent, but do not wish to appeal the decision to the Director, then change the Status of the record to Closed. If we do not hear from you in the next two weeks, we will assume you agree with the WG decision.