Corollary quotient_continuous: forall (X:TopologicalSpace)
  (f g:point_set X -> point_set RTop) (x0:point_set X),
  continuous_at f x0 -> continuous_at g x0 -> g x0 <> 0 ->
  continuous_at (fun x:point_set X => f x / g x) x0 (Y:=RTop).

Lemma Rabs_continuous: continuous Rabs (X:=RTop) (Y:=RTop).