Rav's statement that one who tries to establish his residence under a tree, with an aim to 4,000 amot, hasn't said anything. Namely, the statement is too vague. And not actualizable. The language and the place's identity needed to be more clearly defined. Plus, the tree itself was massive, which makes the location all the more vague. "Underneath the tree" should be clear for a smaller tree, for example. Also, a beraita wherein one who made an eruv in two directions -- becomes problematic. But maybe this stands against Rav's position, in which case, how could Rav counter the beraita (which is from the Tannaim)?! Because he was a Tanna, as it were (Who's Who). Namely, first gen Amora, and a student of R. Yehudah HaNasi. So he's allowed to disagree with that beraita. Now, interspersed in the discussion of Rav's position is a discussion of events that take simultaneously, as compared to sequentially. Whether that's 4 amot of residence, or doubling your tithes, or tithing animals...