We learned in the Mishna (040) that according to Beis Hillel, one must designate birds he would like to slaughter on Yom Tov, before Yom Tov. It was suggested that this is because Beis Hillel does not accept the principle of Bereirah - retroactive clarification - and so the exact birds must be selected before Yom Tov.
Introduction to next subject: We’re discussing a case of Ohel Hameis, there is a room with multiple doorways and windows all of which are closed. Since we know that the meis will be carried out of one of them, we consider them to be part of the ohel and any keilim in the doorway, or on the windowsill is tamei [even though it is on the other side of the door/window], because we consider the door/window to be open. If one door is open, it is considered the exit path for the meis and all other doors and windows are considered closed and keilim there remain tahor.
Challenge: Mishna: If a meis is in a room with a number of entrances [and all are closed (if they’re open, there’s no question that they’re actually a part of the ohel)], all are tamei [keilim in the doorways are all tamei, since each doorway is eligible to be used as the exit]. If one was opened, all are now tahor except for the one which was opened. If he intended to take the meis out from one of the doors or from a window which is 4x4 tefachim [and is thus considered an ‘opening’], it rescues all other openings from becoming tamei.Beis Shammai: This is only if that intention took place before the meis died. Beis Hillel: Even after he died. [So it is clear that Beis Hillel holds of Bereirah!] (The same would hold true if the door was opened, it would then clarify retroactively that only that doorway had become tamei and everything else is tahor. We’re discussing intent which is a bigger chiddush).
Resolution: This Mishna was already explained by Rabbah: To purify the doorways from now on. [Beis Shammai holds that designating a doorway will not change the status of the doorways only actually opening it]. 
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