A sidepost that is visible from the outside, as from within, and R. Yosef says he never heard that halakhah. Who's Who: R. Yosef. Now, the case of a sidepost that ends beyond the wall of the alleyway... works as a lechi, within a certain measurement. If longer, the alleyway is treated as not having any lechi. The next case: A 20-amot-wide alleyway... a couple of suggestions how to divide it and make it usable for an eruv. Also: Another example of R. Dimi's comments, having come from Israel to Babylonia (also Ravin's similar travels), where the discussion of establishing measurements for what amounts to be a toilet, with regard to tumah/taharah, provides an important example of the challenge of "interhalakhicality."