Eruvin 11: Doorways of... Eruvin

Terminology: Tzurat ha-petach. Pitachei shima'ei. Avkata. The need to visualize, and how that plays out, runs throughout the daf. So, what type of space does "tzurat ha-petach" permit? What does it have to look like? How like a door must it be? 2 lechis and a koreh do, fundamentally, create a doorway... Also, the case of an arch, as part of tzurat ha-petach. And: a strange story about R. Nachman, R. Sheshet, R. Gadda, and the Resh Galuta. What is the nature of your arch, on terms of determining its need for a greater distance across (as on this example). Plus: More on the basic case of what needs to be done to turn an alleyway where you can carry (from the new mishnah? Beit Shammai and Beit Hillel disagree, and the discussion moves from the rabbinic space to the Torah concerns of carrying in the public domain.