Ohalot, Chapter Eleven, Mishnah Eight
1) A cellar in a house with a candlestick in it, whose calyx protrudes and an olive-basket is placed such that if the candlestick was taken away the olive-basket would still remain over the mouth of the cellar:
a) Bet Shammai says: the cellar remains clean but the candlestick becomes unclean.
b) Bet Hillel says: the candlestick also remains clean.
2) But they agree that if the olive-basket would fall [into the cellar] if the candlestick was removed, all would become unclean.
Section one: The source of corpse uncleanness is found in the house. The cellar has a candlestick protruding from it, and the calyx (a flower-shaped receptacle on the candlestick) is supporting a basket that covers the opening to the cellar. The basket is of a type that is not susceptible to impurity. If the basket can remain in place without the support of the candlestick, then it joins the walls of the cellar to prevent impurity from going below.
However, Bet Shammai holds that since part of the candlestick protrudes from the cellar, the candlestick is impure. The basket covering it does not protect it from contracting impurity.
Bet Hillel disagrees and holds that the basket protects the candlestick as well.
Section two: If the olive-basket is supported by the candlestick and would collapse without it, it does not protect the impurity from going down below. This is because the candlestick is a vessel and vessels do not act as an ohel to prevent spread of impurity.