rintro ⟨h, hn⟩
case a.mpr
n : ℕ
x : ℕ × ℕ
⊢ x.1 * x.2 = n ∧ ¬n = 0 → ∃ a, (a 0 * a 1 = n ∧ ¬n = 0) ∧ (a 0, a 1) = x
case a.mpr.intro
n : ℕ
x : ℕ × ℕ
h : x.1 * x.2 = n
hn : ¬n = 0
⊢ ∃ a, (a 0 * a 1 = n ∧ ¬n = 0) ∧ (a 0, a 1) = x