clear dec; intros Hl; induction Hl; firstorder congruence.
Qed.
End Dec_in_Prop.
Section Dec_in_Type.
Variables (A:Type)(dec : forall x y:A, {x=y}+{x<>y}).
Definition In_dec := List.In_dec dec. (* Already in List.v *)

(* Specifications: *)
Lemma incl_dec (l l':list A) : {incl l l'}+{~incl l l'}.
Proof using A dec.