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_nat_product: CountableT (nat*nat).