calc
  step n + d'   ≃ _ := step_add
  step (n + d') ≃ _ := Rel.symm add_step
  n + step d'   ≃ _ := AA.substR ‹step d' ≃ d›
  n + d         ≃ _ := ‹n + d ≃ m›
  m             ≃ _ := Rel.refl
case mp
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
left✝ : n ≤ m
right✝ : n ≄ m
d : ℕ
h✝¹ : n + d ≃ m
this✝ : d ≄ 0
this : Positive d
d' : ℕ
h✝ : step d' ≃ d
⊢ step n + d' ≃ m
no goals