apply b₁_of_round_eq
case inl
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₀ : faithful₀ n round
hfaithful₁ : faithful₁ n round
hcorrect₀ : correct₀ n round
hfa : trunc n a = a
hfb : trunc n b = b
hba : b ≤ a
uflow : a - b < 2 ^ n
⊢ trunc n (a - b - round (a - b)) = a - b - round (a - b) ∧ trunc n (round (a - b) - (a - b)) = round (a - b) - (a - b)
case inl.round_eq
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₀ : faithful₀ n round
hfaithful₁ : faithful₁ n round
hcorrect₀ : correct₀ n round
hfa : trunc n a = a
hfb : trunc n b = b
hba : b ≤ a
uflow : a - b < 2 ^ n
⊢ round (a - b) = a - b