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.