[Bug 19504] [QT3TS] instanceof139

https://www.w3.org/Bugs/Public/show_bug.cgi?id=19504

--- Comment #8 from Michael Kay <mike@saxonica.com> ---
>I think things would hang together better if either the anomaly you pointed out regarding A being substitutable for union(A,B) didn't hold,, or if union(A,B) were substitutable for union(A,B,C).  This halfway house leaves things a little incoherent.

I think it makes most sense for our substitutability rules to be the same as
those in XSD. We could allow substitutability in cases where XSD doesn't (as we
do for example with substituting float for double), but it complicates things.

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