Proof with auto.
intros H1 H2. unfold setd.
apply listd_In_pred; [ | auto]; intros e H3.
assert (H4: M.In e cls1 \/ M.In e cls2) by
(rewrite elements_In in H3; apply M.union_spec; auto).
do 2 rewrite <-elements_In in H4.
destruct H4 as [H4|H4];
[apply listd_In_inv_pred with (l := M.elements cls1)|
apply listd_In_inv_pred with (l := M.elements cls2)]...
Qed.

Lemma setd_add c cls s :
setd f inter b cls s -> f c s -> setd f inter b (M.add c cls) s.