Eruvin 48: The Forearm of Og, King of the Bashan

Some inner workings of the beit midrash: Amoraim on tannaitic statements - a need to separate a residential area and a water-cooled ditch. And then R. Yosi bar R. Hanina laughed at the manner of separation... leaving the Gemara to figure out why he was laughing. Also: Among the discussion of how far one has to carry, the Gemara address the basis for "4 amot" - 3 amot for a person's space and an extra amah to bend and maneuver as needed. But are those amot objective distances or specific to each person's own measurements? There are advantages either way, but R. Papa notes how time-consuming it would be to determine objective measurements all the day long. The subjective measurements are, of course, relevant to the maneuvering a person needs with regard to techum and 4 amot. Plus: If you have 2 outer courtyards and one in the middle, the external ones can't carry to the middle one. But that flies in the face of what the eruv is supposed to be doing! So what case would fulfill the details so that it would make sense? To what extent do the participants consider the whole space one joint space? If the eruv itself is divided among the homes, that's not considered joint. Or does being in the same official domain make it a joint eruv?