rewrite [round_eq_trunc_of_le npos hfaithful₁ round_le]
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
round_le : round (a - b) ≤ a - b
⊢ trunc n (round (a - b) - (a - b)) = round (a - b) - (a - b)
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
round_le : round (a - b) ≤ a - b
⊢ trunc n (trunc n (a - b) - (a - b)) = trunc n (a - b) - (a - b)