In the last Mishna (49b) we learned that although a nazir cannot allow himself to become tameh, not all situations of tumat met will force him to begin his nezirut anew.
The Mishna on our daf lists cases where the nazir may formally become tameh met, but he will not need to begin his nezirut over again, rather he will have to wait a week and undergo the process of taharah – purification – after which he will be allowed to resume his nezirut at the point where it was interrupted. Similarly, he will not have to shave his head like a nazir tameh.
Among the cases that appear in the list are a gollel and a dofek, both of which are connected with traditional burial practices, and are, apparently, parts of the tombstone itself.
The commentaries disagree about the definition of gollel and dofek. Rashi explains that the gollel is the cover of a casket, while the dofek refers to the stones upon which the cover rests. Tosafot point out that in several places the Gemara discusses whether an animal can be used as a gollel, and it is difficult to imagine a live animal being used for that purpose. They suggest that the gollel is a rounded stone that was used to close up a burial cave (several such stones have been found near ancient burial caves in Israel).
During the times of the Mishna, common burial practice was to place the dead body in a temporary grave where it would decompose. At a later date, the bones would be removed and transferred to a family burial cave. The round shape of the gollel stone allowed it to be rolled, closing the cave, yet easily opened when necessary. According to this approach, the dofek was the frame upon which the gollel rested.
This image was taken from the English edition of the Koren Talmud Bavli (Steinsaltz Talmud), Tractate Sukka daf 23, page 111.