Let k be given.
rewrite the current goal using
(add_nat_SR m k Hk) (from left to right).
rewrite the current goal using IHk (from left to right).
rewrite the current goal using
(mul_nat_SR n k Hk) (from left to right).
We will
prove n + (n * m + n * k) = n * m + (n + n * k).
We will
prove (n + n * m) + n * k = n * m + (n + n * k).
We will
prove (n * m + n) + n * k = n * m + (n + n * k).
∎