(** * [permutation]: definition and basic properties *)

Definition permutation (l m:list A) := meq (list_contents l) (list_contents m).

Lemma permut_add_inside : forall a l1 l2 l3 l4, permutation (l1 ++ l2) (l3 ++ l4) -> permutation (l1 ++ a :: l2) (l3 ++ a :: l4).