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_fst {U V : UniformSpace} (x : U) (y : V) : continuous (fst (B:=V)) (x, y).