Proof with simpl; auto.
revert cls0; induction l... intros s0 [A C] D; apply IHl...
Qed.

(* Specifications: *)
Lemma setd_fold cls0 l s :
setd f inter b l s -> setd f inter b cls0 s ->
setd f inter b (M.fold (Basics.flip h) cls0 l) s.