Let x and y be given.

Assume Hx Hy.

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

We will prove (x + - x) + y = y.

rewrite the current goal using add_SNo_minus_SNo_rinv x Hx (from left to right).

We will prove 0 + y = y.

An exact proof term for the current goal is add_SNo_0L y Hy.

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