[Haskell-cafe] Fixed point newtype confusion

Hi, Suppose I have the following types:

data Expr expr = Lit Nat | Add (expr, expr) newtype Fix f = Fix {unFix :: f (Fix f)}

I can construct a sample term:

term :: Expr (Expr (Expr expr)) term = Add (Lit 1, Add (Lit 2, Lit 3))

But isn't quite what I need. What I really need is:

term' :: Fix Expr term' = Fix . Add $ (Fix . Lit $ 1, Fix . Add $ (Fix . Lit $ 2, Fix . Lit $ 3))

I feel like there's a stupidly simple way to automatically produce term' from term, but I'm not seeing it. Any ideas? Best, Sebastien