have hab : b ≤ a - b := by
  rewrite [le_tsub_iff_right hba]
  rewrite [← Nat.two_mul]
  exact two_mul_le
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
⊢ 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
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
⊢ trunc n (a - b - round (a - b)) = a - b - round (a - b)