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RE: Postional predicates de-mystified

Subject: RE: Postional predicates de-mystified
From: DPawson@xxxxxxxxxxx
Date: Fri, 4 Jan 2002 09:55:37 -0000
opaque predicate
>  Evan Lenz wrote:

> The predicate in the second example is not part of the Step, 
> but is part of
> a more general FilterExpr. In XPath 1.0, a predicate may 
> follow any kind of
> expression; in this case, it follows a parenthesized expression. The
> parentheses render preceding::foo an expression in its own 
> right (without a
> predicate), yielding an opaque node-set. A node-set never retains
> information about what axis was used to select it. That 
> node-set result is
> subsequently filtered with a predicate. When a predicate applies to an
> expression, the XPath spec says, it is evaluated with respect 
> to the child
> axis. (The child axis is arbitrarily chosen because it's an 
> example of a
> forward axis.) Consequently, the predicate filters out all 
> nodes but the
> first node in *document order*.

Which seems to be the heart of the matter.
A quick scan of xpath 2.0 gives a count of node-set about 7 times (excluding
related occurences).

This word 'sequence' however, appears to occur slightly more times :-)

<quote>A sequence is an ordered collection of zero or more items. An item is
either a simple value or a node.</quote>

The 'order' mentioned here, is it document order or can I still collect a
in reverse document  order?
  what's the plain English definition of a sequence then (as apposed to the

and the FilterExpr has gone from 2.0. Indeed the filter only appears to be
mentioned with
respect to queries. Can I conclude that Evans explanation is no longer
in xpath 2.0, since I can't find an expression of the second form anywhere
in the spec?
(preceding::foo)[1] would now appear to be invalid.

Regards DaveP


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