apply Iff.intro
ℕ : Type
inst✝ : Natural ℕ
n m : ℕ
⊢ Negative (n——m) ↔ (n——m).sgn ≃ -1
case mp
ℕ : Type
inst✝ : Natural ℕ
n m : ℕ
⊢ Negative (n——m) → (n——m).sgn ≃ -1

case mpr
ℕ : Type
inst✝ : Natural ℕ
n m : ℕ
⊢ (n——m).sgn ≃ -1 → Negative (n——m)