specialize hi m n
case hs
r : ↑N0
hi : ∀ (m n : ↑N0), exp (mul (m, n), r) = mul (exp (m, r), exp (n, r))
m n : ↑N0
⊢ exp (mul (m, n), S r) = mul (exp (m, S r), exp (n, S r))
case hs
r m n : ↑N0
hi : exp (mul (m, n), r) = mul (exp (m, r), exp (n, r))
⊢ exp (mul (m, n), S r) = mul (exp (m, S r), exp (n, S r))