rewrite [round_eq_trunc_of_le npos hfaithful₁ round_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
hab : b ≤ a - b
⊢ 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 - trunc n (a - b)) = a - b - trunc n (a - b)