calc
  step n + d   ≃ _ := step_add
  step (n + d) ≃ _ := Rel.symm add_step
  n + step d   ≃ _ := ‹n + step d ≃ m›
  m            ≃ _ := Rel.refl
case h
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✝¹ : n ≤ m
d✝ d : ℕ
x✝ : n + step d ≃ m
⊢ step n + d ≃ m
no goals