Proof using.
induction l; intros; simpl in *; auto.
- constructor.
- constructor; intuition.
match goal with
| H : forall _, _ |- _ => specialize (H a)
end. intuition.
Qed.

Lemma sorted_Permutation_eq :
forall l l',
sorted l ->
sorted l' ->
Permutation l l' ->
l = l'.