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

the : (a : Type) -> a -> a
the _ $x = x

namespace List
  public export
  data List : Type -> Type where
       Nil : List $a
       Cons : $a -> List $a -> List $a

  public export
  length : List $a -> Nat
  length Nil = Z
  length (Cons $x $xs) = S (length xs)

namespace Vect
  public export
  data Vect : Nat -> Type -> Type where
       Nil : Vect Z $a
       Cons : $a -> Vect $k $a -> Vect (S $k) $a

  public export
  length : Vect $n $a -> Nat
  length Nil = Z
  length (Cons $x $xs) = S (length xs)

data Maybe : Type -> Type where
     Nothing : Maybe $a
     Just : $a -> Maybe $a
     
testMaybe : Maybe (Vect ? Integer)
testMaybe = Just (Cons 1 Nil)

data Infer : Type where
     MkInfer : $a -> Infer