rintro ⟨y, ⟨h1, h2⟩, rfl⟩
case a.mp
n : ℕ
x : ℕ × ℕ
⊢ (∃ a, (a 0 * a 1 = n ∧ ¬n = 0) ∧ (a 0, a 1) = x) → x.1 * x.2 = n ∧ ¬n = 0
case a.mp.intro.intro.intro
n : ℕ
y : Fin 2 → ℕ
h1 : y 0 * y 1 = n
h2 : ¬n = 0
⊢ (y 0, y 1).1 * (y 0, y 1).2 = n ∧ ¬n = 0