Definition continuous_on (D : T -> Prop) (f : T -> U) :=
  forall x, D x -> filterlim f (within D (locally x)) (locally (f x)).Definition continuous (f : T -> U) (x : T) :=
  filterlim f (locally x) (locally (f x)).

Lemma continuous_on_subset : forall (D E : T -> Prop) (f : T -> U), (forall x, E x -> D x) -> continuous_on D f -> continuous_on E f.