Open types and down-casting in OCaml

type base_type = ..
class type base = object method base : base_type end

class type a' = object method baba : string method base : base_type end;;
type base_type += A of a'

class a = object (self) method baba = "string" method base = A (self :> a') end

let foo () =
  let a = new a in
  let base = (a :> base) in
  match base#base with
  | A a' -> a'#baba
  | _ -> assert false