have h₀ : b < round (a - round (a - b)) := by
  rewrite [hfe]
  rewrite [lt_tsub_comm]
  exact round_lt
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₀ : faithful₀ n round
hfaithful₁ : faithful₁ n round
hfa : trunc n a = a
hfb : trunc n b = b
hba : b ≤ a
round_lt : round (a - b) < a - b
ulp_le : ulp n a ≤ 2 * b
hfe : round (a - round (a - b)) = a - round (a - b)
⊢ trunc n (a - b - round (a - b)) = a - b - round (a - b)
round : ℕ → ℕ
n a b : ℕ
npos : 0 < n
hfaithful₀ : faithful₀ n round
hfaithful₁ : faithful₁ n round
hfa : trunc n a = a
hfb : trunc n b = b
hba : b ≤ a
round_lt : round (a - b) < a - b
ulp_le : ulp n a ≤ 2 * b
hfe : round (a - round (a - b)) = a - round (a - b)
h₀ : b < round (a - round (a - b))
⊢ trunc n (a - b - round (a - b)) = a - b - round (a - b)