rewrite [round_eq_next_trunc_of_gt hfaithful₁ lt_round]
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 - round (a + b)) = a + b - round (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 (a + b - next n (trunc n (a + b))) = a + b - next n (trunc n (a + b))