Let x and y be given.
Assume Hx Hy.
Use transitivity with (- y) * x, and - (y * x).
An exact proof term for the current goal is mul_SNo_com x (- y) Hx (SNo_minus_SNo y Hy).
An exact proof term for the current goal is mul_SNo_minus_distrL y x Hy Hx.
Use f_equal.
An exact proof term for the current goal is mul_SNo_com y x Hy Hx.
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