final Iterable<? extends T> iterable(@NotNull Function<? super T, ? extends Iterable<? extends T>> tree) {
      return itle != null ? itle : (itle = tree.fun(node)) != null ? itle : JBIterable.empty();
    }
final Iterable<? extends T> iterable(@NotNull Function<? super T, ? extends Iterable<? extends T>> tree)