602 Implausible Expressions

[michaelhkay]

Fix #602

[michaelhkay]

I got the explanation of "disjoint item types" wrong; given two types R and S, the fact that neither is a subtype of the other does not prove they are disjoint. (Consider map(xs:integer, xs:string) and map(xs:decimal, xs:NCName). Indeed, map types of the form map(A,B) and map(X,Y) are never disjoint, because they all have the empty map as an instance. Perhaps I should add that case to the rules.... But pro tem, I think I'll move to an informal definition of "disjoint".

[ndw]

The CG agreed to merge this issue at meeting 043