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Section 3.4.2 talks about the need to union (and intersec) attribute wildcards in complex type extension; section 3.10.6 talks about how attribute wildcards are unioned (and intersected) and mentions the possibility for the result not to be expressible; section 3.4.3 indicates it's an error for the result of "intersection" not to be expressible. But nowhere in the spec talks about what happens when the union is not expressible. A clause 5 is probably in order for 3.4.3: "5 If clause 3.2.2.1.2 in the correspondence specification above for {attribute wildcard} is satisfied, the intensional union must be expressible, as defined in Attribute Wildcard Union (ยง3.10.6)."