Proof using.
induction ys; intros; simpl in *.
- repeat break_match; try discriminate; auto.
- repeat break_match; try discriminate; auto.
intuition.
+ subst. simpl in *. find_inversion. auto.
+ right. repeat find_reverse_rewrite. auto.
Qed.

(* Specifications: *)
Lemma subseq_cons_drop :
forall xs ys (a : A),
subseq (a :: xs) ys -> subseq xs ys.