Inductive CountableT (X:Type) : Prop :=
  | intro_nat_injection: forall f:X->nat, injective f -> CountableT X.Inductive CountableT (X:Type) : Prop :=
  | intro_nat_injection: forall f:X->nat, injective f -> CountableT X.

Lemma countable_exp: forall X Y:Type, FiniteT X -> CountableT Y -> CountableT (X->Y).