// MetaTransformer that understands Transformable and List
sO metaTransformer_transformableAndList() {
  ret new MetaTransformer(
    (o, recurse) -> {
      if (o cast Transformable)
        ret o.transformUsing(recurse);
      null;
    },
    (o, recurse) -> {
      if (o cast L)
        ret map(recurse, o);
      null;
    });
}