Re: Interpretation of (preceding::foo)[1]

preceding::foo[1] parses as a Step not a FilterExpr. Thus 2.1 and 2.4
apply and the axis applicable to the predicate '[1]' is "preceding".

(preceding::foo)[1] parses as a FilterExpr and not a Step. Thus 3.3
applies and the axis applicable to the predicate '[1]' is "child".

"Paul W. Abrahams" wrote:
> 
> The XPath spec, Sec. 3.3.(Node-sets) contains this note:
> 
> The meaning of a Predicate depends crucially on which axis applies.
> For example, preceding::foo[1] returns the first foo element in
> reverse document order, because the axis that applies to the [1]
> predicate is the preceding axis;
> by contrast, (preceding::foo)[1] returns the first foo element in
> document order, because the axis that applies to the [1] predicate is
> the child axis.
> 
> The note seems to be ambiguous in that it doesn't say whether the two
> specifiers are being considered as expressions or as location paths,
> but there are problems of interpretation either way.
> 
> If they are considered as expressions, then preceding::foo[1] is a
> location path treated as an expression while (preceding::foo)[1] is
> directly an expression consisting of the parenthesized expr
> (preceding::foo) narrowed by the predicate[1], and the child axis
> isn't applied, despite what the note says.
> 
> But if they are considered as location paths, then the location path
> (preceding::foo)[1] seems to be an abbreviation of the location path
> child::(preceding::foo)[1], and the selected nodes, order aside, are
> the children of foo nodes rather than the foo nodes themselves.  In
> this case the two node-sets to which the [1] predicate is applied are
> quite different.
> 
> Am I misunderstanding this note or is there really something wrong
> with it?
> 
> Paul Abrahams
> 
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