have htx' : t ≤ y - round (y - x) := calc
  t = y - round (y - z) := ht'
  _ ≤ y - round (y - x) := tsub_le_tsub_left (hml hfz hfx hfy hzx) _
round : ℕ → ℕ
x y z w s t n : ℕ
npos : 0 < n
hfaithful₀ : faithful₀ n round
hfaithful₁ : faithful₁ n round
hml : sub_left_monotonic n round
hfx : trunc n x = x
hfy : trunc n y = y
hfz : trunc n z = z
hfw : trunc n w = w
hzx : z ≤ x
hzy : z ≤ y
hyxulp : ulp n y ≤ 2 * x
ht : t = round (y - round (y - z))
hs₁ : x < t → s = round (x + round (t - x))
hs₂ : t ≤ x → s = round (x - round (x - t))
hxw : x ≤ w
hwy : w ≤ y
hxy : x ≤ y
ht' : t = y - round (y - z)
⊢ trunc n (w - s) = w - s ∧ trunc n (s - w) = s - w
round : ℕ → ℕ
x y z w s t n : ℕ
npos : 0 < n
hfaithful₀ : faithful₀ n round
hfaithful₁ : faithful₁ n round
hml : sub_left_monotonic n round
hfx : trunc n x = x
hfy : trunc n y = y
hfz : trunc n z = z
hfw : trunc n w = w
hzx : z ≤ x
hzy : z ≤ y
hyxulp : ulp n y ≤ 2 * x
ht : t = round (y - round (y - z))
hs₁ : x < t → s = round (x + round (t - x))
hs₂ : t ≤ x → s = round (x - round (x - t))
hxw : x ≤ w
hwy : w ≤ y
hxy : x ≤ y
ht' : t = y - round (y - z)
htx' : t ≤ y - round (y - x)
⊢ trunc n (w - s) = w - s ∧ trunc n (s - w) = s - w