apply trunc_eq_of_le_of_ulp_dvd b
n a b : ℕ
hfa : trunc n a = a
hfb : trunc n b = b
hba : b ≤ a
hab : a ≤ b + b
⊢ trunc n (a - b) = a - b
case hle
n a b : ℕ
hfa : trunc n a = a
hfb : trunc n b = b
hba : b ≤ a
hab : a ≤ b + b
⊢ a - b ≤ b

case hdvd
n a b : ℕ
hfa : trunc n a = a
hfb : trunc n b = b
hba : b ≤ a
hab : a ≤ b + b
⊢ ulp n b ∣ a - b