rewrite [round_eq_trunc_of_le npos hfaithful₁ round_le,
    tsub_eq_zero_of_le (trunc_le n (a + b))]
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
round_le : round (a + b) ≤ a + b
⊢ trunc n (round (a + b) - (a + b)) = round (a + b) - (a + b)
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
round_le : round (a + b) ≤ a + b
⊢ trunc n 0 = 0