[xml] `id` function for XPath

[Daniel Albeseder <daniel albeseder tttech com>]

Hi!

As I could determine from the archive, the `id` function of XPath only
works with the xml:id attribute and the attributes declared as ID inside
the DTD. However attributes of the type xs:ID inside an XML schema does
not work.

This sounds strange, since XML schema is a superset of DTD. Inside the
XPath specification only the DTD is mentioned, however this
specification was created 1999 while the XML schema specification is
from 2001. Therefore I would expect that the ID-mechanism of XML schema
would work also.

Is there are reason, why XML schema does not work with `id`?

Best regards
Daniel