Subject: Re: [xsl] things about grouping From: David Carlisle <davidc@xxxxxxxxx> Date: Wed, 28 Nov 2012 09:47:06 +0000 |
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
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