>So which would you advise to solve the question?
>
> "@cat and not(@cat='BLUE')"
>
> or
> "@cat!='BLUE'"

I'd choose to use the first... as it reads easier.  Basically stating:
 My results must have the attribute "cat" and the value of this
attribute can not equal "BLUE".

So in all that we've learned, the != is basically a shorthand version
of this?  As well, you can think of it like this:  you have to satisfy
the left in order to satisfy the right.  (just making sure I
understand)

Karl