We prove the intermediate
claim H0lt_y:
0 < y.
rewrite the current goal using H0eqx (from left to right) at position 1.
An exact proof term for the current goal is Hxlt.
rewrite the current goal using H0eqx (from right to left).
An
exact proof term for the current goal is
(mul_SNo_pos_pos y y HyS HyS H0lt_y H0lt_y).
∎