induction sz <;> intros n m
sz : ℕ
⊢ ∀ (n m : FinInt sz), toUint (addfull n m) = toUint n + toUint m
case zero
n m : FinInt Nat.zero
⊢ toUint (addfull n m) = toUint n + toUint m

case succ
n✝ : ℕ
n_ih✝ : ∀ (n m : FinInt n✝), toUint (addfull n m) = toUint n + toUint m
n m : FinInt (Nat.succ n✝)
⊢ toUint (addfull n m) = toUint n + toUint m