Eruvin 9: When Like Finds Like

Topics recurring with more development on each pass, like a spiral passing through again, but at a distance reflecting that development (as an observation of organization). With a focus on Lavud, today - the extension of the material to cover a gap before the wall. As long as that gap is less than 3 tefachim (or 4), you don't need to add an additional koreh (crossbeam). What about a fully suspended beam, with gaps on both sides? In that case, it seems too much to expect lavud to work. Though the Gemara seems to indicate that it would after all. Who's Who: Rabbi Zakkai. The case? For a "makom patur," carrying under the beam, but not between two sideposts. But there are different opinions here. How many sideposts are necessary to establish the area underneath the koreh and between the two lechis?! And what about only one lechi? Isn't that area akin to the public domain? The Gemara answers, No! And uses biblical allusion to do so. The karmelit essence extends itself, type going after type, as it were. Basically, extending rabbinic jurisdiction.