(** Restriction of a filter to a domain *)
Definition within {T : Type} D (F : (T -> Prop) -> Prop) (P : T -> Prop) :=
F (fun x => D x -> P x).
Global Instance within_filter :
forall T D F, Filter F -> Filter (@within T D F).

Lemma filter_le_within :
forall {T} {F : (T -> Prop) -> Prop} {FF : Filter F} D,
filter_le (within D F) F.