Reference for Eruvin 104:11

ר"א אומר שתים יכנס שלש לא יכנס: והתניא ר"א אומר אחת יכנס שתים לא יכנס לא קשיא הא דעקר חדא וקם אתרתי הא דעקר תרתי וקם אתלת

refers to a person who left the first cubit but was still within the second,<span class="x" onmousemove="('comment',' 'TWO CUBITS' implying that the man walked to within two cubits which he has not completely traversed.');">¹⁹</span> while the latter refers to one who left the second and was within the third.<span class="x" onmousemove="('comment',' 'Two cubits' in the Baraitha having the meaning that the man walked across the two cubits and was thus already within the third one.');">²⁰</span>

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