@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