Let x and y be given.

Assume Hx Hy.

rewrite the current goal using mul_SNo_com x y Hx Hy (from left to right).

rewrite the current goal using mul_SNo_com x (- y) Hx (SNo_minus_SNo y Hy) (from left to right).

An exact proof term for the current goal is mul_SNo_minus_distrL y x Hy Hx.

∎