[F&O] fn:doc and fragment-ids

I'm am confused as to what is supposed to happen if the argument to
fn:doc() includes a fragment-id.

On the one hand, the draft says the argument is an xs:anyURI, passed
as an xs:string, so it is legitimate to include a fragment-id.
On the other hand, the semantics is defined as a mapping of strings
onto DOCUMENT nodes, and a fragment-id cannot point to a document node
(though it might point to a document element).

But the way URIs are mapped onto document nodes is not specified. Does
this mean it is implementation-defined as to whether the fragment-id
is used, or is it an error to include one?
-- 
Colin Paul Adams
Preston Lancashire