apply ih
ℕ : Type
inst✝² : Core ℕ
inst✝¹ : Induction ℕ
inst✝ : Addition ℕ
n✝ m k n : ℕ
ih : n + m ≃ n + k → m ≃ k
x✝ : step n + m ≃ step n + k
⊢ m ≃ k
ℕ : Type
inst✝² : Core ℕ
inst✝¹ : Induction ℕ
inst✝ : Addition ℕ
n✝ m k n : ℕ
ih : n + m ≃ n + k → m ≃ k
x✝ : step n + m ≃ step n + k
⊢ n + m ≃ n + k