>   On the other hand, it seems (if I understand correctly the
> code) that the optimizer will rewrite:
> 
>     xxx and my:user-function(yyy)
> 
> into:
> 
>     if (xxx) then my:user-function(yyy) else false()
> 
>   I don't understand why this rewrite is intended to

It makes the code potentially tail-recursive.

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