rewrite [tsub_eq_zero_of_le $ Nat.le_of_lt $ lt_next_trunc npos _]
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
lt_round : a - b < round (a - b)
⊢ trunc n (a - b - next n (trunc n (a - b))) = a - b - next n (trunc n (a - b))
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
lt_round : a - b < round (a - b)
⊢ trunc n 0 = 0