revert b
ℕ : Type
inst✝ : Natural ℕ
s b : Difference ℕ
n m : ℕ
⊢ (n——m).sgn ≃ s → b.sgn ≃ s → (n——m + b).sgn ≃ s
ℕ : Type
inst✝ : Natural ℕ
s : Difference ℕ
n m : ℕ
⊢ ∀ {b : Difference ℕ}, (n——m).sgn ≃ s → b.sgn ≃ s → (n——m + b).sgn ≃ s