## Re: [xsl] Re: Re: order of UNIONs

 Subject: Re: [xsl] Re: Re: order of UNIONs From: Jörg Heinicke Date: Thu, 15 Nov 2001 08:39:58 +0100
```Thanks, Dimitre, for the explanations. I must confess, that I don't know
much about node-sets in theory, only things I came across when using them in
XSL. Are there any nice websites or books (Michael Kay's?) about node-set
theory?

Joerg

> > But does this really make sense? select="group[21]|group[1]" means for
me
> > 'select the 21st group and add the 1st group'.
>
> It makes ***perfect sense***. Node-sets are sets. Sets do not have order.
Regardless
> of the order, in which you add elements, the result is the same set:
>
> a | b  =  b | a
>
> The above is an axiom in set theory.
>
> What you actually need is kinda bag, or list -- these are very different
structures
> >from sets. Both allow duplicates, and a list has order.
>
> I think one of the major problem of the XPath 2.0 Data Model is that they
do not
> distinguish between lists and sets, trying somehow to say that a node-set
is a kind
> of list (the actual term used there was "sequence", if I remember well).
The
> consequences are bad problems, because some operations on lists cannot be
performed
> 1:1 on sets, and vice versa -- not every list is a set, an operation
performed on a
> sequence, that is a nodeset, may not yield a nodeset, a nodeset operation
performed
> on a sequence that is a nodeset, may behave quite differently from the
same
> operation, performed on a sequence (e.g. eliminating/preserving duplicates
on a
> union/append operation).
>
> Cheers,
> Dimitre Novatchev.

XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list

```