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[Bug 24569] Least common types and lattices

From: <bugzilla@jessica.w3.org>
Date: Fri, 07 Feb 2014 09:31:19 +0000
To: public-qt-comments@w3.org
Message-ID: <bug-24569-523-6nB2a9Aase@http.www.w3.org/Bugs/Public/>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=24569

--- Comment #3 from Michael Kay <mike@saxonica.com> ---
The only XSD union types that are item types are "pure union types" as defined
in 

http://www.w3.org/TR/xpath-30/#dt-pure-union-type

I believe it is a consequence of the rules in

http://www.w3.org/TR/xpath-30/#id-itemtype-subtype

that given two union types U = union(A, B) and V = union(B, A), then U and V
are "equal" in the type graph, that is, they are subtypes of each other and
they have the same supertypes and subtypes, even though they are distinct in
the sense that they define different cast operations on strings.

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