data Bool : Type where
     False : Bool
     True : Bool

not : Bool -> Bool
not False = True
not True = False

data Nat : Type where
     Z : Nat
     S : Nat -> Nat

plus : Nat -> Nat -> Nat
plus Z y = y
plus (S k) y = S (plus k y)

data Vect : ? -> Type -> Type where
     Nil : Vect Z a
     Cons : a -> Vect k a -> Vect (S k) a

data Pair : Type -> Type -> Type where
     MkPair : a -> b -> Pair a b

zip : Vect n a -> Vect n b -> Vect n (Pair a b)
zip Nil Z impossible
zip Nil Nil = Nil
zip (Cons x xs) (Cons y ys) = Cons (MkPair x y) (zip xs ys)