calc
  step n * m ≃ _ := step_mul
  n * m + m  ≃ _ := AA.substL ih
  m * n + m  ≃ _ := Rel.symm mul_step
  m * step n ≃ _ := Rel.refl
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 : ℕ
ih : n * m ≃ m * n
⊢ step n * m ≃ m * step n
no goals