fn:document-uri returns the empty sequence.
fn:document-uri will only return the URI of a document
when a document node is passed into it. If you want to return the URI
of a node that is not a document node, but has a document node ancestor,
use fn:base-uri or
xdmp:node-uri.
var x = cts.doc("/bar.json");
fn.documentUri(x);
=> "/bar.json"