have hac : a ≤ round (a + b) := calc
  a = trunc n a       := hfa.symm
  _ ≤ trunc n (a + b) := trunc_le_trunc npos (Nat.le_add_right _ _)
  _ ≤ round (a + b)   := trunc_le_round _ _ hfaithful₁
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₀ : faithful₀ n round
hfaithful₁ : faithful₁ n round
hcorrect₁ : correct₁ n round
hfa : trunc n a = a
hfb : trunc n b = b
hba : b ≤ a
lo : round (a + b) ≤ a + b
⊢ round (a + b) + round (b - round (round (a + b) - a)) = a + b
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₀ : faithful₀ n round
hfaithful₁ : faithful₁ n round
hcorrect₁ : correct₁ n round
hfa : trunc n a = a
hfb : trunc n b = b
hba : b ≤ a
lo : round (a + b) ≤ a + b
hac : a ≤ round (a + b)
⊢ round (a + b) + round (b - round (round (a + b) - a)) = a + b