> Yes, I did it for keeping the (large) pattern in one place 
> instead of two, and 
> for optimization, to have it evaluated only once. But that's perhaps 
> unnecessary?
> 

It's good practice, I think, to use a variable in such cases. Some
optimizers may recognize common subexpressions, but there's no point relying
on it.

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