> Or be prepared for all kinds of a nasty surprise following the fact
> that the value of
>    
>        my:f($x) is my:f($x)
> 
> is generally not guaranteed to be true()

Yes indeed. The folks writing the formal semantics have been having fun with
this - as have the folks writing optimizers, and those writing test suites
;-)

It's not really any worse than the fact that (in XQuery notation) (<a/> is
<a/>) return false. In fact, the problem is even there in XSLT 1.0.

Michael Kay