case mpr =>
  intro (_ : a ≃ b)
  show a - b ≃ 0
  calc
    a - b  ≃ _ := AA.substL ‹a ≃ b›
    b - b  ≃ _ := sub_same
    0      ≃ _ := Rel.refl
ℕ : Type
inst✝⁶ : Natural ℕ
ℤ : Type
inst✝⁵ : Core ℤ
inst✝⁴ : Addition ℤ
inst✝³ : Multiplication ℤ
inst✝² : Negation ℤ
inst✝¹ : Sign ℤ
inst✝ : Subtraction ℤ
a b : ℤ
⊢ a ≃ b → a - b ≃ 0
no goals