RE: why is "(chapter//footnote)[1]" illegal?

>   boning up on my predicates and patterns, i'm reading
> kay, p. 443, which states:
> 
>   "(chapter//footnote)[1] is not a valid pattern.  (Why not?
> No good reason, it's just that the spec doesn't allow it."
> 
>   but on p. 408, there is an explanation of the (apparently
> acceptable) path expression "(chapter/para)[1]".  
> 
>   so is it just the difference between using the child axis
> and the descendant-or-self axis?  it's not obvious to me
> why the first should be illegal while the second is legal.

The syntax for patterns is a small subset of the syntax for XPath
expressions, and the subset doesn't allow parentheses.

The subset was chosen to make it easy for implementations to test
whether a node matches a pattern without going through the full
algorithm of evaluating the expression for every ancestor of the node
being tested.

Michael Kay


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