Definition inj_on_list {A B} (l : list A) (f : A -> B) :=
  forall x y, In x l -> In y l -> f x = f y -> x = y.Definition subset_rel {X} (sub lst : list X) : Prop :=
  forall x,
    In x sub ->
    In x lst.

Lemma NoDup_app_remove_mid : forall (A : Type) (l2 l1 l3 : list A) , NoDup (l1 ++ l2 ++ l3) -> NoDup (l1 ++ l3).