(* Impredicative or, which can emulate a classical not-implies. *)
Definition unless condition property : Prop :=
 forall goal : Prop, (condition -> goal) -> (property -> goal) -> goal.Notation "{ 'on' cd , 'bijective' f }" := (bijective_on (mem cd) f)
  (at level 0, f at level 8,
   format "{ 'on'  cd ,  'bijective'  f }") : type_scope.

Lemma unless_contra b C : implies (~~ b -> C) (\unless C, b).