have h' := FinInt.addfull_toUint n' m'
sz : ℕ
n' m' : FinInt sz
br : Bool
r : FinInt sz
h : addfull n' m' = next br r
⊢ toSint
      (match next br r with
      | next false r => next false (next (true != false) r)
      | next true r => next true (next (true == false) r)) =
    match
      match next br r with
      | next false r => next false (next (true != false) r)
      | next true r => next true (next (true == false) r) with
    | next false a => toSint (next true n') + toSint (next false m') + 2 ^ (sz + 1)
    | next true a => toSint (next true n') + toSint (next false m') - 2 ^ (sz + 1)
sz : ℕ
n' m' : FinInt sz
br : Bool
r : FinInt sz
h : addfull n' m' = next br r
h' : toUint (addfull n' m') = toUint n' + toUint m'
⊢ toSint
      (match next br r with
      | next false r => next false (next (true != false) r)
      | next true r => next true (next (true == false) r)) =
    match
      match next br r with
      | next false r => next false (next (true != false) r)
      | next true r => next true (next (true == false) r) with
    | next false a => toSint (next true n') + toSint (next false m') + 2 ^ (sz + 1)
    | next true a => toSint (next true n') + toSint (next false m') - 2 ^ (sz + 1)