Our addition function is (Y R).
We can work through this by hand and compute
((Y R) (SUCC ZERO) (SUCC ZERO))
The result is indeed (SUCC (SUCC ZERO))
It's totally straightforward
I'd do it now, but it won't fit on the slide.
It's tedious but instructive to work it out for yourself
If you do it, you'll see how the miraculous Y works