Definition subfamily (f:family) (D:R -> Prop) : family :=
mkfamily (intersection_domain (ind f) D) (fun x y:R => f x y /\ D x)
(restriction_family f D).
Definition compact (X:R -> Prop) : Prop :=
forall f:family,
covering_open_set X f ->
exists D : R -> Prop, covering_finite X (subfamily f D).

(* Specifications: *)
(**********)
Lemma family_P1 :
forall (f:family) (D:R -> Prop),
family_open_set f -> family_open_set (subfamily f D).