Section subseq.
Variable A B : Type.
Hypothesis A_eq_dec : forall x y : A, {x = y} + {x <> y}.
Proof using.
induction l; simpl; tauto.
Qed.
Proof using.
induction zs; intros; simpl in *;
repeat break_match; subst; simpl in *; intuition; subst; eauto;
right; (eapply IHzs; [|eauto]); simpl; eauto.
Qed.
Proof using.
induction ys; intros.
- destruct xs; simpl in *; intuition.
- simpl in *. break_match; simpl in *; intuition; subst; intuition eauto;
right; (eapply IHys; [eauto| intuition]).
Qed.

Theorem subseq_NoDup :
forall (ys xs : list A),
subseq xs ys ->
NoDup ys ->
NoDup xs.