intro b hi a
case hs

⊢ ∀ (n : ↑N0),
    (∀ (a : ↑N0), mul (S a, n) = plus (mul (a, n), n)) → ∀ (a : ↑N0), mul (S a, S n) = plus (mul (a, S n), S n)
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)