Chapter 2, Mishna 4
Another strategy for adjusting a karpef, non-residential enclosure, which is just over the 5000 square cubit size limit for carrying on Shabbos: reduce its size. We can’t do that by adding trees, but we can place a column there that’s 10 tefachim, handbreadths, high and has an area of 4 square handbreadths. This is its own private domain, and it reduces the enclosure’s area by the same amount, bringing it within the maximum size for carrying on Shabbos. We can also build an inner fence as long as it’s far enough away from the old one to make a significant change. Commentators point out that we can look at this process two ways: reducing the enclosure’s size or building a new fence to re-designate the enclosure as residential. We can’t do that by building the new fence atop the old one. We can acquire ownerless land by enclosing it or plowing it, but not by scattering seeds on it. A Jewish convert is a full-fledged Jew in every way, with a spiritual ancestry tracing back to Abraham and Sarah. If he acquired land in ancient Israel and died without children, his non-Jewish relatives could not inherit that land, hence the question about acquiring ownerless land. What happens if a karpef adjoins a residential courtyard and the wall between them falls down? What if a karpef adjoins a house and the wall of the house falls down?