have ⟨d, (_ : n₁ + d ≃ n₂)⟩ := le_defn.mp ‹n₁ ≤ n₂›
ℕ : Type
inst✝³ : Core ℕ
inst✝² : Induction ℕ
inst✝¹ : Addition ℕ
inst✝ : Sign ℕ
order_inst : Order ℕ
n₁ n₂ : ℕ
x✝ : n₁ ≤ n₂
⊢ step n₁ ≤ step n₂
ℕ : Type
inst✝³ : Core ℕ
inst✝² : Induction ℕ
inst✝¹ : Addition ℕ
inst✝ : Sign ℕ
order_inst : Order ℕ
n₁ n₂ : ℕ
x✝ : n₁ ≤ n₂
d : ℕ
h✝ : n₁ + d ≃ n₂
⊢ step n₁ ≤ step n₂