Reference for Eruvin 27:6

רבן שמעון בן גמליאל אומר פחות מד' אין צריך להביא קורה אחרת ארבע צריך להביא קורה אחרת

R'Shesheth said: If one laid a beam across [an entrance to] an alley and spread a mat over it, raising [the lower end of the mat to a height of] three handbreadths from the ground, there is here neither valid cross-beam nor valid partition. There is here no valid cross-beam, since it is covered up; and no valid partition, since it is one through which kids can push their way.<span class="x" onmousemove="('comment',' A suspended partition of such a character is invalid in an alley.');">⁸</span>

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