rewrite [← round_eq_next_trunc_of_gt hfaithful₁ lt_round]
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₀ : faithful₀ n round
hfaithful₁ : faithful₁ n round
hfa : trunc n a = a
hba : 2 * b ≤ a
lt_round : a - b < round (a - b)
hba' : b ≤ a
h₁ : a ≤ b + round (a - b)
h₂ : a ≤ round (a - b) + b
h₃ : round (a - b) ≤ a
⊢ next n (trunc n (a - b)) - (a - b) ≤ b
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₀ : faithful₀ n round
hfaithful₁ : faithful₁ n round
hfa : trunc n a = a
hba : 2 * b ≤ a
lt_round : a - b < round (a - b)
hba' : b ≤ a
h₁ : a ≤ b + round (a - b)
h₂ : a ≤ round (a - b) + b
h₃ : round (a - b) ≤ a
⊢ round (a - b) - (a - b) ≤ b