## Re: [xsl] Understanding Identity Transformations

 Subject: Re: [xsl] Understanding Identity Transformations From: Karl Stubsjoen Date: Sat, 12 Feb 2005 15:26:08 -0700
```More good input!  (still trying to digest that... but it is making sense)

On Sat, 12 Feb 2005 22:00:49 -0000, Michael Kay <mike@xxxxxxxxxxxx> wrote:
> > > Wow.. that is easy except that I do not understand your notice
> > > explanation... the difference between your not( ... ) and your !=.
> >
> > "@cat != 'BLUE'": will be true if the context node has a
> > 'cat' attribute
> > *and* if it has a 'cat' attribute not equal to 'BLUE'. So
> > it's the same as
> > "boolean(@cat) and not(@cat='BLUE')"
> >
> > "not(@cat='BLUE')": will be true if the context node has no 'cat'
> > attribute *or* if it has a 'cat' attribute not equal to
> > 'BLUE'. So it's
> > equal to "not(@cat) or not(@cat='BLUE')"
>
> Let's try to phrase it a different way. An expression such as X=Y in XPath
> is shorthand for
>
> some \$x in X, \$y in Y satisfies \$x=\$y
>
> in other words, it's true if there's some pair of values from the two sets
> that are equal.
>
> Similarly, X!=Y is short for
>
> some \$x in X, \$y in Y satisfies \$x!=\$y
>
> which is true if there's some pair that are not equal.
>
> This means that if X is an empty set, then X=3 and X!=3 must both be false.
> If you're testing an attribute, @A=3, then @A is a set that's either empty
> or contains one node. If there's no A attribute, then @A!=3 is false
> (because there's no A that's not equal to 3), but not(@A=3) is true (because
> it's not true that there's an A that's equal to 3).
>
> Michael Kay
> http://www.saxonica.com/

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