:- use_module(library(http/html_write)). xhtml_ns(+Id, 
+Value)
xhtml_ns(+Id, 
+Value)//xmlns 
channel. Rdfa (http://www.w3.org/2006/07/SWD/RDFa/syntax/), 
embedding RDF in (x)html provides a typical usage scenario where we want 
to publish the required namespaces in the header. We can define:
rdf_ns(Id) -->
        { rdf_global_id(Id:'', Value) },
        xhtml_ns(Id, Value).
After which we can use rdf_ns//1 as a 
normal rule in html//1 to publish 
namespaces from library(semweb/rdf_db). Note that this 
macro only has effect if the dialect is set to xhtml. In
html mode it is silently ignored.
The required xmlns receiver is installed by html_begin//1 
using the html tag and thus is present in any document that 
opens the outer html environment through this library.