calc
  n * m ≃ _ := AA.substR ‹m ≃ 0›
  n * 0 ≃ _ := mul_zero
  0     ≃ _ := Rel.refl
case mpr.right
ℕ : Type
inst✝⁵ : Core ℕ
inst✝⁴ : Induction ℕ
inst✝³ : Addition ℕ
inst✝² : Sign ℕ
inst✝¹ : Order ℕ
inst✝ : Multiplication ℕ
n m : ℕ
x✝¹ : n ≃ 0 ∨ m ≃ 0
x✝ : m ≃ 0
⊢ n * m ≃ 0
no goals