Proof using.
intros f P Q PQ s e. induction e as [s ok | x s e induc_hyp].
- destruct s as (x, s); simpl in *. rewrite map_Cons. constructor 1.
rewrite <- map_Cons. apply PQ. exact ok.
- rewrite map_Cons. constructor 2. exact induc_hyp.
Qed.
(* The converse seems to require much more work *)
Definition fstAB := fst (A:=A) (B:=B).

Lemma exteq_fst_zip:
forall sA sB, exteq (map fstAB (zip sA sB)) sA.