{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeFamilies #-}

import Data.Kind (Type)

data Nat = Z | S Nat

data family QuadTree :: Nat -> Type -> Type

newtype instance QuadTree  Z    a = Leaf a
data    instance QuadTree (S n) a = Branch (QuadTree n a) (QuadTree n a)
                                           (QuadTree n a) (QuadTree n a)