Corollary sum_continuous: forall (X:TopologicalSpace)
  (f g:point_set X -> point_set RTop) (x:point_set X),
  continuous_at f x -> continuous_at g x ->
  continuous_at (fun x:point_set X => f x + g x) x (Y:=RTop).

Lemma Rminus_continuous: continuous_2arg Rminus (X:=RTop) (Y:=RTop) (Z:=RTop).