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