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

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

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

append : Vect $n $a -> Vect $m $a -> Vect (plus $n $m) $a
append Nil $ys = ys
append (Cons $x $xs) $ys = Cons x ?foo