code: public boolean isDefaultNamespace(String namespaceURI) {
          int type = getNodeType();
          if (type == NodeKind.ATTR) {
              if (this instanceof AttrImpl == false) {
                  // ns decl
                  throw new UnsupportedOperationException();
              }
          }
          Node p = getParentNode();
          return p.isDefaultNamespace(namespaceURI);
      }
docs: {@inheritDoc}
  <p>
  Not supported for namespace declaration. Overrided by DocumentImpl and
  ElementImpl
  </p>