[XQuery] IBM-XQ-025: Comparable types in Order By clause

(IBM-XQ-025) Section 3.9.3, Order By and Return Clauses: The first 
bulleted list says that each orderspec in an Order By clause must "return 
values of the same type for all tuples." We should use a different term: 
"comparable types" rather than "same type", and we should define the term. 
For the purposes of this rule, all numeric types should be "comparable". 
The types xs:string and xdt:untypedAtomic should be "comparable". Any 
atomic type should be "comparable" with its derived types, and two atomic 
types that are derived from the same base type (or that are both derived 
from numeric types) should be "comparable".

--Don Chamberlin