code: public void setNamespace(String prefix, String namespaceURI) {
      ensureNotNull("Prefix", prefix);
      ensureNotNull("Namespace URI", namespaceURI);
  
      namespaces.put(prefix, namespaceURI);
    }
docs: Maps a single prefix, uri pair as namespace.
  
  @param prefix the prefix to use
  @param namespaceURI the URI to use
  @throws IllegalArgumentException if prefix or namespaceURI is null