Talmud Bava Batra 18b  The Gemara first brings the Mishna that that the sages say to keep the mustard away from the bees and that is  a question on Rava that says something that causes damage must be kept away from the boundary even if there is nothing on the other side that could be damaged.  Then it answers the question telling us that the Mishna is just saying mustard can cause damage but the law of Rava still stands that even when there are no bees still one can not put mustard next to the border.
Then the Gemara asks from R. Yose. R. Yose says it is permitted because the owner of the mustard can tell the owner of the bees why tell me to keep my mustard away? You should keep your bees away because they cause damage to my mustard.

To R. Tam (and R. Kananel), the gemara answers this thus "Ravina said the sages hold the one that causes damage must keep the object that cases damage away from the boundary." (I mean to say that this answer is a different answer than that of Rav Papa.)

The way R Tam explains it is thus. At first we thought the sages held the one that causes damage must keep his object away. and now after the answer we think he must keep his object away only when there s something on the other side of the boundary that could be damaged. That is, that Rava retracted his statement.  My question is then what does this have to do with the question they were asking from R Yose? In the Gemara we apparently took care of the first part of the Mishna. It was from R Yose that we were asking