intro (n——m)
ℕ : Type
inst✝ : Natural ℕ
⊢ ∀ (a : Difference ℕ), AA.ExactlyOneOfThree (a ≃ 0) (Positive a) (Negative a)
ℕ : Type
inst✝ : Natural ℕ
n m : ℕ
⊢ AA.ExactlyOneOfThree (n——m ≃ 0) (Positive (n——m)) (Negative (n——m))