cases br <;> simp at *
sz : ℕ
n' m' : FinInt sz
h✝ : True
br : Bool
r : FinInt sz
h_nov :
  (match next br r with
      | next msb r' => (next msb r', msb)).snd =
    false
h : next false n' + next false m' = next br r
⊢ toSint
      (match next br r with
        | next msb r' => (next msb r', msb)).fst =
    toSint (next false n') + toSint (next false m')
case false
sz : ℕ
n' m' : FinInt sz
h✝ : True
r : FinInt sz
h : next false n' + next false m' = next false r
h_nov : True
⊢ toSint (next false r) = toSint (next false n') + toSint (next false m')