code: public UnprotectedStringBuffer getNamespaceURI(CharSequence namespacePrefix) {
  		for (ElementContext ctx : event.context) {
  			for (Pair<UnprotectedStringBuffer, UnprotectedStringBuffer> ns : ctx.namespaces)
  				if (ns.getValue1().equals(namespacePrefix))
  					return ns.getValue2();
  		}
  		return new UnprotectedStringBuffer();
  	}
docs: Get the namespace URI for a prefix, or empty string if the prefix is not defined.