I'm curious about what's going on behind the scenes (that is, in the
processor) that makes this expensive.
without having looked behind the scenes, on the face of it count(.|B) is
a very expensive operation (if "B" selects a large set of noders) as you
have to take the set union | which invloves removing duplicates etc,
then count the thing, which means that you need to fully evaluate all of
B.
All you actually need to know is is . in the set returned by B which you
can ask in xpath 2 using the is operator (so it would be a lot more
efficient even if except wasn't added as a primitive.
With such a test you can stop and return true as soon as you have found
the current node, just as B[1] typically only causes a processor to
evaluate one node, even though the formal semantics of the expression
are that B is fully evaluated, sorted into document order, and then the
first item is taken.
David
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