calc
  m            ≃ _ := Rel.symm ‹step n + k ≃ m›
  step n + k   ≃ _ := step_add
  step (n + k) ≃ _ := Rel.symm add_step
  n + step k   ≃ _ := Rel.refl
case mp.right
ℕ : Type
inst✝³ : Core ℕ
inst✝² : Induction ℕ
inst✝¹ : Addition ℕ
inst✝ : Sign ℕ
order_inst : Order ℕ
n m : ℕ
x✝ : n < m
this : step n ≤ m
k : ℕ
h✝ : step n + k ≃ m
⊢ m ≃ n + step k
no goals