Require Export FiniteTypes Relation_Definitions ZArith QArith IndexedFamilies.
FunctionalExtensionality ProofIrrelevance
DependentTypeChoice ClassicalChoice
Arith ArithRing.
Local Close Scope Q_scope.
Set Asymmetric Patterns.
Inductive CountableT (X : Type) : Prop :=
| intro_nat_injection (f : X -> nat) : injective f -> CountableT X.
Definition Countable {X : Type} (S : Ensemble X) : Prop :=
CountableT {x:X | In S x}.

Lemma inj_countable {X Y : Type} (f : X -> Y) :
CountableT Y -> injective f -> CountableT X.