@gplx.Virtual
 public void Render(Mustache_bfr bfr, Mustache_render_ctx ctx) {
   throw Err_.new_unsupported();
 } // should be abstract
 @gplx.Virtual
 public void Subs_ary_(Mustache_tkn_itm[] v) {
   throw Err_.new_unsupported();
 } // fail if trying to set and not overridden