Reference for Eruvin 179:5

בעי רב ביבי בר אביי ב' אמות בגג וב' אמות בחורבה מהו

is forbidden<span class="x" onmousemove="('comment',' Because, presumably, they belong to different tenants.');">¹⁷</span> so is also that between a roof<span class="x" onmousemove="('comment',' Of a dwelling-house.');">¹¹</span> and an exedra<span class="x" onmousemove="('comment',' Sc. the roof of an exedra, that did not belong to the owner of the adjoining roof and house, that was bigger than two beth se'ah, that had no partitions around it, that was in a sloping position and that had in consequence the status of a karmelith,');">¹²</span> forbidden.<span class="x" onmousemove="('comment',' For the same reason (cf. prev. n.) .');">¹⁸</span> R'Bebai B'Abaye enquired: Is it permissible to move an object two cubits on a roof and two cubits in a ruin?<span class="x" onmousemove="('comment',' That belonged to a different owner, and that had the status of a karmelith because one of its sides was completely exposed to a public domain.');">¹⁹</span>

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