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.

Lemma subseq_trans :
forall (zs xs ys : list A),
subseq xs ys ->
subseq ys zs ->
subseq xs zs.