Reference for Eruvin 153:11

ואי אשמעינן בחריץ משום דלא בעיתא תשמישתא אבל כותל דבעיתא תשמישתא אימא לא צריכא

And [the enunciation of] both cases<span class="x" onmousemove="('comment',' Those of 'wall' and 'trench'.');">²⁸</span> was required. For if we had been informed only of the law of the wall<span class="x" onmousemove="('comment',' Permitting the use of the top of the wall.');">²⁹</span>

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