use fun i => if i = 0 then x.fst else x.snd
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
case h
n : ℕ
x : ℕ × ℕ
h : x.1 * x.2 = n
hn : ¬n = 0
⊢ (((if 0 = 0 then x.1 else x.2) * if 1 = 0 then x.1 else x.2) = n ∧ ¬n = 0) ∧
    (if 0 = 0 then x.1 else x.2, if 1 = 0 then x.1 else x.2) = x