data Expr f = In (f (Expr f))
data Val e = ValC Int
type IntExpr = Expr Val

-- λ> In (ValC 3)
-- In (ValC 3) :: Expr Val

data Add e = AddC e e
type AddExpr = Expr Add 

-- In (_: Add (Expr Add))
-- In $ AddC (_: Expr Add) (_: Expr Add)

-- λ> In (AddC (In (ValC 2)) (In (ValC 3)))

data (f :+: g) e
  = Inl (f e)
  | Inr (g e)

type IntAddExpr = Expr (Val :+: Add)
-- In (Inr _)
-- In (Inr AddC (In (Inl (ValC 2)))
--              (In (Inl (ValC 3))))


a = In (Inr (AddC (In (Inl (ValC 2))) (In (Inl (ValC 3)))))
b = In $ Inr $ AddC (In (Inl (ValC 2))) (In (Inl (ValC 3)))
c = In $ Inr $ AddC (In $ Inl $ ValC 2) (In $ Inl $ ValC 3)