induction l as [|a l IH].
- left; now constructor.
- destruct (In_dec a l).
+ right. inversion_clear 1. tauto.
+ destruct IH.
* left. now constructor.
* right. inversion_clear 1. tauto.
Qed.
End Dec_in_Type.
(** An extra result: thanks to decidability, a list can be purged from redundancies. *)

Lemma uniquify A (d:decidable_eq A)(l:list A) :
exists l', NoDup l' /\ incl l l'.