Section Perm.
Variable A : Type.
Hypothesis eq_dec : forall x y:A, {x=y} + {~ x=y}.
Notation permutation := (permutation _ eq_dec).
Notation list_contents := (list_contents _ eq_dec).
(** we can use [multiplicity] to define [In] and [NoDup]. *)
(** When used with [eq], this permutation notion is equivalent to the one defined in [List.v]. *)
(** Permutation for short lists. *)

Lemma permut_length_2 :
forall a1 b1 a2 b2, permutation (a1 :: b1 :: nil) (a2 :: b2 :: nil) ->
(a1=a2) /\ (b1=b2) \/ (a1=b2) /\ (a2=b1).