class Foo {
    abstract static class Animal<A extends Animal<A>> {
          abstract A copyAnimal(A animal);
    }

    static class Duck<D extends Duck<D>> extends Animal<D> {
      void quack() {}

      D copyAnimal(D duck) {
        duck.quack();
        return duck;
      }
    }

    static class Mallard extends Duck<Mallard> {
      Mallard copyAnimal(Mallard duck) {
        duck.quack();
        return duck;
      }
    }
    
    public static void main(String[] args) {}
}