Let z be given.
Assume Hz.
Let x be given.
Assume Hx.
Let y be given.
Assume Hy.
Assume Hzxy: z = pa x y.
rewrite the current goal using Hzxy (from left to right).
Apply CSNo_I to the current goal.
An exact proof term for the current goal is real_SNo x Hx.
An exact proof term for the current goal is real_SNo y Hy.
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