Question on uri generation in N3

I'd like to know what techniques are available in N3 for generating uri's.

The way that I figured out is as follows:

{?x a ace:Document. ("http://www.congo.org/book#" ?x.ace:Document_ID) string:concatenation ?y. ?z log:uri ?y} => {?z a congo:Book.}.

Does N3 provide any other alternatives for geneating uri's?

Is there a way to use a prefix (by itself) in a rule?  Along the lines below (which doesn't work).

@prefix congo: <http://www.congo.org/book#>.
{?x a ace:Document. (<congo> ?x.ace:Document_ID) string:concatenation ?y. ?z log:uri ?y} => {?z a congo:Book.}.

> Regards,
> 
> David H. Jones
> Boeing Phantom Works, 
> Mathematics & Computing Technology
> 425-865-6924 
> 425-865-2964 (FAX)
> david.h.jones@boeing.com
> 
>