have : a//1 * b//1 ≃ (a * b)//1 := calc
  a//1 * b//1      ≃ _ := eqv_refl
  (a * b)//(1 * 1) ≃ _ := substD AA.identL
  (a * b)//1       ≃ _ := eqv_refl
ℕ : Type
inst✝¹ : Natural ℕ
ℤ : Type
inst✝ : Integer ℤ
a b : ℤ
⊢ (a * b)//1 ≃ a//1 * b//1
ℕ : Type
inst✝¹ : Natural ℕ
ℤ : Type
inst✝ : Integer ℤ
a b : ℤ
this : a//1 * b//1 ≃ (a * b)//1
⊢ (a * b)//1 ≃ a//1 * b//1