RE: XSLT 2.0 empty string

> Is the error XT0570?
> 
> [ERR XT0570] It is a type error if the supplied value of a 
> variable cannot be converted to the required type.

Yes. 
> 
> How should we read the '?' in this case?
>  Convert to string if you can, else don't bother?

The ? is part of the type. It means that the value (after atomization) must
either be a string, or nothing (an empty sequence).

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