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In paragraph 2.5.6.1 ( http://www.w3.org/TR/xquery-30/#id-seqtype-subtype ) it is said that subtype(Ai*, Bi?) always returns false. If we do support the predefined XML Schema 1.1 simple type xs:error which is a union type without member types, should there not be an exception in the case where Ai is a pure union type with no member types? The same applies for Ai+ and Bi.
See https://lists.w3.org/Archives/Member/w3c-xsl-query/2013Mar/0263.html
The Working Group accepted comment #1, I will add a note saying this is primarily for the sake of xs:error.
I have changed that cell from: false to: false (exception: <code>true</code> when Ai is a pure union type with no member types) Note: xs:error is a pure union type with no member types; this is the main motivation for the exception noted in the above table.