abstract class Fruit:
      prop name: string
      prop rotten: bool

    class Apple (Fruit):
      prop name: string = "apple"
      prop rotten: bool = False

    class Orange (Fruit):
      prop name: string = "orange"
      prop rotten: bool = False

    func add (a: List<Fruit>, b: List<Fruit>) -> List<Fruit>:
      return foldl(lambda l, f: l.append(f), a, b)

    func filter (p: Func<[Fruit], bool>, l: List<Fruit>) -> List<Fruit>:
      return [f for f in l if p(f)]

    func is_rotten (f: Fruit) -> bool:
      return f.rotten

    a1 = Apple()
    a2 = Apple()
    a3 = Apple()
    a3.rotten = True
    apples = [a1, a2, a3]

    o1 = Orange()
    o2 = Orange()
    o3 = Orange()
    o3.rotten = True
    oranges = [o1, o2, o3]

    fruits = add(apples, oranges)

    print(len(filter(is_rotten, apples)), "/", len(apples))
    > "1/3"

    print(len(filter(is_rotten, oranges)), "/", len(oranges))
    > "1/3"

    print(len(filter(is_rotten, fruits)), "/", len(fruits))
    > "2/6"