Proof using.
induction xs; intros; simpl.
- auto.
- repeat break_match; auto.
+ intuition congruence.
+ find_inversion. auto.
Qed.

(* Specifications: *)
Lemma subseq_map :
forall (f : A -> B) ys xs,
subseq xs ys ->
subseq (map f xs) (map f ys).