Proof using.
induction l1; intros.
- auto.
- simpl ((a :: l1) ++ l2) in *.
find_apply_lem_hyp cons_contiguous_sorted; auto.
simpl in *. intuition.
Qed.

(* Specifications: *)
Lemma prefix_sorted :
forall l l',
sorted l ->
Prefix l' l ->
sorted l'.