have : sgn ((a:ℚ) - (b:ℚ)) ≃ -1 := calc
  _ ≃ sgn ((a:ℚ) - (b:ℚ)) := Rel.refl
  _ ≃ sgn (((a - b):ℤ):ℚ) := sgn_subst (eqv_symm sub_compat_from_integer)
  _ ≃ sgn (a - b)         := sgn_from_integer
  _ ≃ -1                  := Integer.lt_sgn.mp ‹a < b›
ℕ ℤ ℚ : Type
inst✝¹⁰ : Natural ℕ
inst✝⁹ : Integer ℤ
inst✝⁸ : Core ℚ
inst✝⁷ : Addition ℚ
inst✝⁶ : Multiplication ℚ
inst✝⁵ : Negation ℚ
inst✝⁴ : Sign ℚ
inst✝³ : Subtraction ℚ
inst✝² : Reciprocation ℚ
inst✝¹ : Division ℚ
inst✝ : Order ℚ
a b : ℤ
x✝ : a < b
⊢ from_integer a < from_integer b
ℕ ℤ ℚ : Type
inst✝¹⁰ : Natural ℕ
inst✝⁹ : Integer ℤ
inst✝⁸ : Core ℚ
inst✝⁷ : Addition ℚ
inst✝⁶ : Multiplication ℚ
inst✝⁵ : Negation ℚ
inst✝⁴ : Sign ℚ
inst✝³ : Subtraction ℚ
inst✝² : Reciprocation ℚ
inst✝¹ : Division ℚ
inst✝ : Order ℚ
a b : ℤ
x✝ : a < b
this : sgn (from_integer a - from_integer b) ≃ -1
⊢ from_integer a < from_integer b