(** [list_disjoint l1 l2] holds iff [l1] and [l2] have no elements
  in common. *)

Definition list_disjoint (A: Type) (l1 l2: list A) : Prop :=
  forall (x y: A), In x l1 -> In y l2 -> x <> y.

Lemma list_disjoint_sym: forall (A: Type) (l1 l2: list A), list_disjoint l1 l2 -> list_disjoint l2 l1.