Reference for Eruvin 153:12

בא למעטו אם יש במיעוטו ארבעה מותר להשתמש בכל הכותל כולו ואם לאו אין משתמש אלא כנגד המיעוט

it might have been assumed to apply to it alone, because people make use of a raised structure, but not to a trench, since people do not make use of a depression in the ground.<span class="x" onmousemove="('comment',' And its use is, therefore, despite its comparatively low altitude, forbidden to the tenants of both courtyards.');">³⁰</span> And if we had been informed of the law of the trench only<span class="x" onmousemove="('comment',' Cf. supra n. 9 mut. mut.');">³¹</span> it might have been assumed to apply to it alone, because its use involves no anxiety<span class="x" onmousemove="('comment',' Since any object put into it remains safely in its position.');">³²</span>

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