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