Proof using.
induction ys; intros; simpl in *; repeat break_match; auto; try discriminate; intuition; subst.
- simpl. find_rewrite. auto.
- auto using subseq_skip.
- auto using subseq_nil.
- simpl. find_rewrite. auto.
Qed.

(* Specifications: *)
Lemma subseq_app_r :
forall xs ys,
subseq (A:=A) ys (xs ++ ys).