> not(A!=B) is true just if A!=B is false which is only the 
> case if every
> node in A is equal to every node in B. If that's what you want to test
> for then the expression is put to good use, but it's rather 
> rare to have
> a node set where all the nodes are string equal (unless it's something
> like @abc where you know there is at most one node in the set).

There are of course use cases for !=. For example

if (//@xml:lang != 'en')

tests if the document contains an xml:lang attribute whose value is not
equal to "en".

But they are rather rare in practice.

Michael Kay
http://www.saxonica.com/