# RE: RE: Selecting the first node set

 Subject: RE: RE: Selecting the first node set From: cknell@xxxxxxxxxx Date: Tue, 15 Aug 2006 11:07:44 -0400
```I never got a direct answer to my question about the meaning of the parentheses, but I think I've worked it out.

I find it easier to think of things visually (I always did well in geometry but stumbled in algebra). What we're really working with is the notion of sets, so I think of a Venn diagram. I picture this:

"x | y[1]"

as "The set of all x-things and the first y-thing."

and this:

"(x | y)[1]"

as "The first element of the set of all x-things and all y-things."

--
Charles Knell
cknell@xxxxxxxxxx - email

-----Original Message-----
From:     Wendell Piez <wapiez@xxxxxxxxxxxxxxxx>
Sent:     Tue, 15 Aug 2006 10:31:59 -0400
To:       xsl-list@xxxxxxxxxxxxxxxxxxxxxx
Subject:  RE:  Selecting the first node set

At 01:00 PM 8/14/2006, Mike wrote:
> > One has to be willing to think with the
> > literal-mindedness of a machine to tell the difference
> > between /descendant-or-self::node()/child::Value[1] and
> > (/descendant-or-self::node()/child::Value)[1], and that can
> > take a bit of practice.
>
>Yes. I'm not sure why people struggle with this though. It seems intuitive
>enough that
>
>x | y[1]
>
>doesn't mean the same as
>
>(x | y)[1]
>
>so why is the "/" perceived so differently from the "|"? Is it that people
>are somehow aware that it's a higher-order operator and therefore imagine it
>doesn't obey normal precedence rules? Or is it the overloading of [], which
>in its other role as a boolean filter is associative with "/"?

Possibly all of the above, and more, but none of it in specific. I
think it's really a case of brain overloading. When faced with what
appear to be complex expressions that remind them of middle-school
examinations, people's anxiety levels rise and their neural pathways
tighten up, leading to a negative-feedback-loop-driven brain shutdown
and a reversion to what "ought" to be. Some people learn to relax,
slow down and let the channels open, or they have learned from
practice not to be scared by mathematics, or they were never scared
in the first place.

It's a fascinating thing to observe. As a teacher, I try to convey
that all this persnicketiness is not only necessary (to understand
the machine, which doesn't suffer from anxiety), but it can be fun,
not scary, and that no one's hand will be slapped. (Of course, that
runs the risk of me being classed as the Other, at which point
teaching becomes even more of a challenge. :-)

But you've given me an idea. Maybe our poor school-scarred students
would be helped by examining

/ descendant-or-self::node() / child:element[1]

as a gloss on //element[1]

Thanks again Mike.

Cheers,
Wendell

>Michael Kay
>http://www.saxonica.com/

```

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