C0p question about the lecture

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Why is it on the last step, we use Subst and exec together?

is there no one understand this? i would really like some help…

Well, I recommend doing the detailed derivation of → like in example 6.3.9.

Screenshot 2022-05-24 at 19.27.58

Because i think, in the last step in the photo that i posted. Its enough using exec and assign. Why should ae use subst?

Again, try drawing a derivation tree using just Assign and Exec. You will notice it is not possible. More specifically, the reason is there are nested blocks.