specialize hi a
case hs
b : ↑N0
hi : ∀ (a : ↑N0), mul (S a, b) = plus (mul (a, b), b)
a : ↑N0
⊢ mul (S a, S b) = plus (mul (a, S b), S b)
case hs
b a : ↑N0
hi : mul (S a, b) = plus (mul (a, b), b)
⊢ mul (S a, S b) = plus (mul (a, S b), S b)