daf bit: Bava Kama 53

Along with the ox that gores, the torah talks about people who dig pits and don't cover them to prevent people and animals from falling in. Today's daf talks about compounded liability for pits. If a man puts a stone near the mouth of a pit that was dug by someone else, and an ox stumbles over the stone and thereby falls into the pit, who pays damages? Rava says Rabbi Natan says the owner of the pit pays; while the person who placed the stone shares liability, the owner of the ox can say to the owner of the pit "I found my dead ox in your pit", and this is sufficient even though the owner of the pit can say "if not for the stone the ox would not have fallen in at all" (to which the person who placed the stone could say "if not for the pit, tripping over my stone would have done no harm"). (53a)

(The discussion here is a little confusing. I think I have the attributions right.)