have ⟨d, (_ : n + d ≃ m)⟩ := le_defn.mp ‹n ≤ m›
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
⊢ step n ≤ m
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✝ : n + d ≃ m
⊢ step n ≤ m