Well XPath has direct support for union and intersect so for
completeness we would expect it to support difference. Thats
reasonable isn't it. So lets go to the part of the specification that
deals with set theoretic operators and see what it says.

The first hint of trouble is in the URL.

http://www.w3.org/TR/xpath-functions/#union-intersection-except

Lets read on and see why there is support for union and intersection
and not difference.

op:union 	Returns the union of the two sequence arguments, eliminating
duplicates.
op:intersect 	Returns the intersection of the two sequence arguments,
eliminating duplicates.
op:except 	Returns the difference of the two sequence arguments,
eliminating duplicates.

So difference is supported except (pun intended) it is called except.

But (sic) the semantic of except  that has been ingrained in us since
childhood is that when you say  A except B, B takes it context from A.

A|B
A intersect B
A except B
A,B

________________________________________________________________________
The Numerical Algorithms Group Ltd is a company registered in England
and Wales with company number 1249803. The registered office is:
Wilkinson House, Jordan Hill Road, Oxford OX2 8DR, United Kingdom.