Definition point_adherent (D:R -> Prop) (x:R) : Prop :=
  forall V:R -> Prop,
    neighbourhood V x ->  exists y : R, intersection_domain V D y.Definition adherence (D:R -> Prop) (x:R) : Prop := point_adherent D x.

Lemma included_trans : forall D1 D2 D3:R -> Prop, included D1 D2 -> included D2 D3 -> included D1 D3.