Definition setd {T} (f : M.elt -> T) (g : T -> T -> T) (b : T) (s : M.t) :=
  listd f g b (M.elements s).

Lemma listd_fold_left_wk cls0 l s : listd (fun c => forall s, f c s) and True l -> listd f inter b cls0 s -> listd f inter b (fold_left h l cls0) s.