show from_prod (n * k + m * j, n * j + m * k)
   ≃ from_prod (k * n + j * m, k * m + j * n)
ℕ : Type
inst✝ : Natural ℕ
n m k j : ℕ
⊢ (n * k + m * j)——(n * j + m * k) ≃ (k * n + j * m)——(k * m + j * n)
ℕ : Type
inst✝ : Natural ℕ
n m k j : ℕ
⊢ from_prod (n * k + m * j, n * j + m * k) ≃ from_prod (k * n + j * m, k * m + j * n)