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