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What is the basis for the ruling that adding to a tent is permissible on Shabbath?

The Gemara in 'Eruving 102a quotes Rav as telling Rav Huna that in order to provide shade for his flock in the daytime on Shabbath yet roll back the cover in the evening for ventilation, all he has to do is, when he rolls back the protective cover, leave a Tefah unfurled, so that there is a partial covering at all times, and that way when he unfurls it the rest of the way he is merely extending the cover, not creating a "tent" on Shabbath.

This, I assume, is the basis for the common Heter for people to have tarp covers on their Sukkoth, which they keep rolled almost all the way when they want the Sukkah uncovered, but leave a Tefah unfurled, and then totally roll out when they need to protect the Sukkah (or really its contents) from rain.

But what's the basis for the ruling? There's nothing that Rav quotes. There's a story just before it about Remi bar Yehezkel asking R' 'Amram to tell him a fascinating thing in the name of R' Asi, which, when R' 'Amram responds, sounds somewhat similar. But is that it? Rav just tells R' Huna to do this and it gets codified in the Gemara?