[Bug 19504] [QT3TS] instanceof139

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

--- Comment #7 from Tim Mills <tim@cbcl.co.uk> ---
Since a value's dynamic type will never be a union type, but rather one of the
union's member types, using derives-from with AT (actual type) being a union
type seems a tad strange.

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.

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