Rava and R. Yosef needed to set up an eruv, so Rava plans to set up a residence under a specific tree. R. Yosef doesn't know which tree, but Rava assures him that his familiarity is enough. And he credits R. Yosi as the tannaitic authority upon whom he relies for this. It's not clear, however, whether R. Yosi had anything to do with this view. Plus: The source for 2,000 amot for eruv techumin, and many-verse chain of links to get there.