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The rules in section 6.5 (Default Priority of Template Rules) should be reviewed for new ItemTypes introduced in XPath 3.0: function items union types maps There's a fallback rule that gives these all a priority of 0.5, but it's not clear this is the best we can do. For a union type I would suggest "the minimum priority of the atomic types in the transitive membership of the union, minus 0.5." This gives union(date, dateTime) a lower priority than dateTime but a higher priority than anyAtomicType, and it gives union(ID, IDREF) a lower priority than IDREF but a higher priority than NCName. We specify the default priority for ~function(*) as -1; I suggest that any more specific function type should have priority zero, ~map(*) should have +0.5, and the default priority of ~map(K,V) should be the default priority of K.
The proposal was accepted.
The changes have been applied in editor's draft K.