rewrite [ulp_eq, ← Nat.mod_add_mod,
    Nat.mod_eq_zero_of_dvd (ulp_dvd_of_trunc_eq hfa), Nat.zero_add]
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
hcorrect₁ : correct₁ n round
hfa : trunc n a = a
no_uflow : 2 ^ n ≤ a + b
no_carry : a + b < 2 ^ Nat.size a
lt_round : a + b < round (a + b)
size_eq : Nat.size (a + b) = Nat.size a
ulp_eq : ulp n (a + b) = ulp n a
ulp_le : ulp n a ≤ 2 * b
⊢ ulp n (a + b) - (a + b) % ulp n (a + b) ≤ b
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
hcorrect₁ : correct₁ n round
hfa : trunc n a = a
no_uflow : 2 ^ n ≤ a + b
no_carry : a + b < 2 ^ Nat.size a
lt_round : a + b < round (a + b)
size_eq : Nat.size (a + b) = Nat.size a
ulp_eq : ulp n (a + b) = ulp n a
ulp_le : ulp n a ≤ 2 * b
⊢ ulp n a - b % ulp n a ≤ b