diff --git a/library/data/countable.lean b/library/data/countable.lean
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+import data.fintype data.list data.sum data.nat
+open option list nat
+
+structure countable [class] (A : Type) :=
+(pickle : A → nat) (unpickle : nat → option A) (picklek : ∀ a, unpickle (pickle a) = some a)
+
+open countable
+
+definition countable_fintype [instance] {A : Type} [h₁ : fintype A] [h₂ : decidable_eq A] : countable A :=
+countable.mk
+  (λ a, find a (elements_of A))
+  (λ n, nth (elements_of A) n)
+  (λ a, find_nth (fintype.complete a))
+
+definition countable_nat [instance] : countable nat :=
+countable.mk (λ a, a) (λ n, some n) (λ a, rfl)
+
+definition countable_option [instance] {A : Type} [h : countable A] : countable (option A) :=
+countable.mk
+  (λ o, match o with
+        | some a := succ (pickle a)
+        | none := 0
+        end)
+  (λ n, if n = 0 then some none else some (unpickle A (pred n)))
+  (λ o,
+    begin
+    cases o with [a],
+      begin esimp end,
+      begin esimp, rewrite [if_neg !succ_ne_zero, pred_succ, countable.picklek] end
+    end)
+
+section sum
+variables {A B : Type}
+variables [h₁ : countable A] [h₂ : countable B]
+include h₁ h₂
+
+definition pickle_sum : sum A B → nat
+| (sum.inl a) := 2 * pickle a
+| (sum.inr b) := 2 * pickle b + 1
+
+definition unpickle_sum (n : nat) : option (sum A B) :=
+if n mod 2 = 0 then
+   match unpickle A (n div 2) with
+   | some a := some (sum.inl a)
+   | none   := none
+   end
+else
+   match unpickle B ((n - 1) div 2) with
+   | some b := some (sum.inr b)
+   | none   := none
+   end
+
+open decidable
+theorem unpickle_pickle_sum : ∀ s : sum A B, unpickle_sum (pickle_sum s) = some s
+| (sum.inl a) :=
+  assert aux : 2 > 0, from dec_trivial,
+  begin
+    esimp [pickle_sum, unpickle_sum],
+    rewrite [mul_mod_right, if_pos (eq.refl 0), mul_div_cancel_left _ aux, countable.picklek]
+  end
+| (sum.inr b) :=
+  assert aux₁ : 2 > 0,       from dec_trivial,
+  assert aux₂ : 1 mod 2 = 1, by rewrite [modulo_def],
+  assert aux₃ : 1 ≠ 0,       from dec_trivial,
+  begin
+    esimp [pickle_sum, unpickle_sum],
+    rewrite [add.comm, add_mul_mod_self_left aux₁, aux₂, if_neg aux₃, add_sub_cancel_left,
+             mul_div_cancel_left _ aux₁, countable.picklek]
+  end
+
+definition countable_sum [instance] {A B : Type} [h₁ : countable A] [h₂ : countable B] : countable (sum A B) :=
+countable.mk
+  (λ s, pickle_sum s)
+  (λ n, unpickle_sum n)
+  (λ s, unpickle_pickle_sum s)
+
+end sum