newtype Id a = Id a

newtype Const a b = Const a

data Pair f fs a = Pair (f a) (fs a)

data Unary f fs a = Z (fs (fs a)) | S (Unary f (Pair f fs) a)

type Square = Unary Id (Const ())

zero :: Square a
zero = Z (Const ())

one :: Square Int
one = S (Z (Pair (Id (Pair (Id 1) (Const ()))) (Const ())))

two :: Square Int
two = S . S . Z $ Pair (Id $ Pair (Id 1) $ Pair (Id 2) $ Const ())
                $ Pair (Id $ Pair (Id 3) $ Pair (Id 4) $ Const ())
                $ Const ()