Michael Kay wrote:

The operation you are referring to is usually called "set difference". In
XPath 2.0, the operator is spelt "except".  There is also a set 
intersection
operator spelt "intersect". There's no equivalent to these in XSLT 1.0,
though some processors support the EXSLT set:difference() and
set:intersection() functions. You can expensively code 

A except B =>

   A[count(.|B) != count(B)]

A intersect B =>

   A[count(.|B) = count(B)]

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Hi, Mike,

If you don't mind waxing theoretical for a minute, what makes those 
comparisons expensive?

I'm curious about what's going on behind the scenes (that is, in the 
processor) that makes this expensive.

Thanks.

Jay Bryant
Bryant Communication Services
(presently consulting at Synergistic Solution Technologies)