have := decode_list (e.unpair.2)
case some
β : Type u_1
inst✝ : Encodable β
e : ℕ
this : e.unpair.2 < e + 1
b : β
⊢ Option.map (fun x => b :: x) (decode e.unpair.2) =
    (mapM' decode (Denumerable.ofNat (List ℕ) e.unpair.2)).bind fun __do_lift => some (b :: __do_lift)
case some
β : Type u_1
inst✝ : Encodable β
e : ℕ
this✝ : e.unpair.2 < e + 1
b : β
this : decode e.unpair.2 = mapM' decode (Denumerable.ofNat (List ℕ) e.unpair.2)
⊢ Option.map (fun x => b :: x) (decode e.unpair.2) =
    (mapM' decode (Denumerable.ofNat (List ℕ) e.unpair.2)).bind fun __do_lift => some (b :: __do_lift)