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MB4 200a: Carrying from One Domain to Another on Shabbos (346:2-3)
Carrying 4 amot in a public domain is an issur d'oraita. Chazal extended the prohibition to a karmelit. Carrying from one karmelit to another (less than 4 amot).
Status of a קרפף and a man-made mechitza vs. a natural mechitza.