> I recently needed to do set difference operation in a 2.0 
> stylesheet, using Saxon 9 (basic version). I was a bit 
> surprised to find that Saxon reported, "Unknown system 
> function except()".
> 
> The F&O spec defines this function
> (http://www.w3.org/TR/xquery-operators/#func-except). Could 
> you please tell, why this built in function not implemented 
> in Saxon 9.

It's not a function, it's an operator. (Note the op: prefix). You can write
"A except B".
> 
> As a workaround, I had to resort to $seq1[not(. = $seq2)] ...
> 

That doesn't do the same thing, it compares by value, not by node identity. 

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