The gemarah posits that the machlokes between abaye and R' Yosef is in whether or not they hold like R' Ami... that without a 4 tefach piece of wall off the korah the enclosure is not b'etzem a mavui.
Abaye asks on R' Yosef that it can't be that the min shiur of a mavui is 4 tfachim because then wouldn't be able to have the doors aopening into it (necessity for mavui)
R' Yosef answers that there can still be a 4 tefach opening because it is diagonal off the keren zavis (on the back side, not the korah side, so we don't run into the problem that an opening by a keren zavis undermines the pesach) and there is still a peice of wall off the korah so you now have a minimum mavuiand your 4 tefach doors.
Question: R' Yosef's answer to Abaye is in the tzad that R' Yosef holds like R' Ami and R' Asi to get his minimum shiur of Daled Tefachim. But that din is derived fro the fact that under 4 tefachim of wall by the korah undermine the mavui status. So how can R' Yosef answer w/ keren zavis, where even if the pesachim are diagonal, it still leaves at most about only 1 tefach of wall off the korah?
moshe rubin, brooklyn, New York
It is possible that Rav Yosef understood that Rebbi Ami's Halachah regarding the four Tefachim at the beginning of the Mavuy merely gave the minimum amount of closed space which the beginning of a Mavuy must have in order to avoid having a Pirtzah. However, this does not mean that when there is no Pirtzah that one must have four closed Tefachim at the beginning of his Mavuy. As long as one would have four Tefachim of any sort, which includes openings for Batim and Chatzeiros, this would be considered a Hechsher Mavuy according to Rav Yosef at this stage of the Gemara.
that works except (as I understand it) as far as the picture in Rashi goes, the diagonals are just representing where the chatzeros start, but there are no actual doors. If there were doors I would understand, but as I imagine it looked, as far as the mavui is concerned there is no difference between this case and if there were a korah with a tefach or so of wall, and then a pirtzah less than 10.....
(if understood the answer wrong i am sorry, it just seems to be a sticking point)
I don't know why you would prefer to learn the Gemara's assumption as being difficult (tefach, plus Pirtzah less than ten Amos), instead of learning the Gemara as stated above, that the Gemara's assumption is that we are only dealing with a Mavuy with Pesachim of four Tefachim, which is the simple understanding of the Gemara.
In any event, if you would understand the Gemara better with doors, the Rashash indeed says that it is possible that the case is where there are doors that open backwards into the Chatzer (he also says that it is possible that there are no doors at all).