For was it not stated: If an exedra was situated in a valley
Rab ruled, it is permitted to move objects within all its interior (on Shabbat);
but Samuel ruled: Objects may be moved within four cubits only.
Rab ruled that it was permitted to move objects in all its interior, because we apply [the principle:] ‘The edge of the ceiling descends and closes up.’ But Samuel ruled that objects may be moved within four cubits only, because we do not apply [the principle:] ‘The edge of the ceiling descends and closes up?’
At dispute is the idea that a suspended roof can create ‘virtual walls’ which define the space.
Roman architecture and virtual space combine!