Bava Batra 5: Ravina vs. Ronya
The case of an inner all and an outer wall - can one party induce the other the participate in payment for fencing just by virtue of putting them up - a court case between Ravina and Ronya. Also, a new mishnah! On the obligation when a courtyard wall collapses and neighbors all need to pay in to construct a new wall, of at least 4 amot high. Always, the one who claims he has contributed needs to prove it. The Gemara introduces a time factor - when the debt comes due.