Let m be given.
rewrite the current goal using
(mul_nat_SR n m Hm) (from left to right).
rewrite the current goal using
(mul_nat_SL m Hm n Hn) (from left to right).
We will
prove n + n * m = m * n + n.
rewrite the current goal using IHm (from left to right).
We will
prove n + m * n = m * n + n.
∎