[F&O] IBM-FO-020: Need a function to dereference XPointers

(IBM-FO-020) Section 15.4, Functions and Operators that Generate 
Sequences: We need a function that takes an XPointer as its parameter, 
dereferences it, and returns the node referenced by the XPointer. Since 
this function has similarities to both fn:doc and fn:id, it should be 
defined in the same section as these functions. This function would 
satisfy XQuery Requirement 3.3.4, which requires support for references 
from one XML document to another.

--Don Chamberlin