intro a b
⊢ ∀ (a b : ℕ), (if a = 0 ∨ b = 0 then 0 else ↑1) = if b = 0 then 0 else (if a = 0 then 0 else ↑1) * ↑1
a b : ℕ
⊢ (if a = 0 ∨ b = 0 then 0 else ↑1) = if b = 0 then 0 else (if a = 0 then 0 else ↑1) * ↑1