apply Nat.dvd_sub hba
case hdvd
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
hfa : trunc n a = a
hfb : trunc n b = b
round_le : round (a - b) ≤ a - b
two_mul_le : 2 * b ≤ a
ulp_le : ulp n (a - b) ≤ b
hba : b ≤ a
hab : b ≤ a - b
⊢ ulp n b ∣ a - b
case hdvd.h₁
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
hfa : trunc n a = a
hfb : trunc n b = b
round_le : round (a - b) ≤ a - b
two_mul_le : 2 * b ≤ a
ulp_le : ulp n (a - b) ≤ b
hba : b ≤ a
hab : b ≤ a - b
⊢ ulp n b ∣ a

case hdvd.h₂
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
hfa : trunc n a = a
hfb : trunc n b = b
round_le : round (a - b) ≤ a - b
two_mul_le : 2 * b ≤ a
ulp_le : ulp n (a - b) ≤ b
hba : b ≤ a
hab : b ≤ a - b
⊢ ulp n b ∣ b