type-union functions

Why are these functions defined as binary operators?  (XACML 3.0-wd-06
A.3.11 Set functions.)

Consider defining a 3.0 version that accepts zero or more bags of the
same type.  This would be a marginal improvement in ease of use and
clarity of expression.

Instead of:
	(string-union (string-union bag1 bag2) bag3)

I could write:
	(string-union bag1 bag2 bag3)

--Paul Tyson