Proof using.
intros. induction l; simpl in *; intuition.
break_if; do_bool; simpl in *; intuition.
congruence.
Qed.

(* Specifications: *)
Lemma sorted_Prefix_in_eq :
forall l' l,
sorted l ->
Prefix l' l ->
(forall e, In e l -> In e l') ->
l' = l.