Rif Succah 11a {Succah 22b continues; 23a; 24b - 25a; 26b}

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11a

{22b, Mishna, continues}
If two {walls} were made by human hands and one was a tree {such that it supports it}, or two by tree and one by human hands, it is valid and one may not ascend to it on Yom Tov.
Three by human hands and one a tree, it is valid and one may ascend to it on Yom Tov.

{Succah 23a}
This is the rule: Anything which if you take the tree away it is able to stand on its own is valid, and one may ascend to it on Yom Tov.

{Succah 24b}
One who makes his succah between trees and the trees are walls for it, it is valid.

{Succah 25a}
Agents fulfilling a commandment {shluchei mitzvah} are exempt from Succah. Sick people and their attendants are exempt from succah.

{Succah 26b}
One may eat and drink non-permanently outside the succah.

There was an incident in which they brought before Rabban Yochanan ben Zakkai to taste the cooked dish, and to Rabban Gamliel two dates and a jar