have hihi : next n (trunc n b) ≤ next n (trunc n a) :=
  Nat.add_le_add lolo (ulp_le_ulp n lolo)
n a b : ℕ
round : ℕ → ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
hcorrect₀ : correct₀ n round
hcorrect₁ : correct₁ n round
hba : b ≤ a
lolo : trunc n b ≤ trunc n a
lohi : trunc n b ≤ next n (trunc n a)
⊢ round b ≤ round a
n a b : ℕ
round : ℕ → ℕ
npos : 0 < n
hfaithful₁ : faithful₁ n round
hcorrect₀ : correct₀ n round
hcorrect₁ : correct₁ n round
hba : b ≤ a
lolo : trunc n b ≤ trunc n a
lohi : trunc n b ≤ next n (trunc n a)
hihi : next n (trunc n b) ≤ next n (trunc n a)
⊢ round b ≤ round a