What exactly happened? How did Rav Huna bar Chinena intend to permit
carrying in the orchard by erecting the poles, and what was Rava's
objection?
(a) RASHI gives two explanations. In his first explanation, he writes that
an Avarneka is not a place of real residential use (Hukaf l'Dirah); people
do not live there, they only relax there. Rav Huna bar Chinena placed poles
in the ground in the form of an enclosed area in order to represent a little
room in which a guard could live (i.e., a Dirah). By doing so, he intended
to make the entire orchard "Hukaf l'Dirah," (i.e., its outer wall now
included a residential area as well, and not just an orchard) and to permit
carrying in the orchard.
Rava pulled out the poles because he felt that the little makeshift room in
the orchard does not make the orchard into a place that was Hukaf l'Dirah,.
One reason the little room does not help is because it was erected *after*
the orchard was fenced in. Second, it is only temporary and not made for
living in. Third, it is made primarily for protecting items placed in it,
and its main purpose is not for residence.
According to this explanation, Rav Huna bar Chinena made a mistake in the
Halachos of Hukaf l'Dirah when he attempted to permit carrying in the
orchard by erecting the poles.
(b) In his second explanation, RASHI explains that the Avarneka was indeed a
place for living in to some extent, since people stay within it for extended
periods of time. It is a room of some sort (and not just a shady tree over a
bench as in Rashi's first explanation). Rav Huna bar Chinena held that since
the Avarneka was built after the orchard was enclosed, the Avarneka does not
make the entire orchard to be considered Hukaf l'Dirah. Therefore, he
erected poles in the arrangement of a *path from the house to the Avarneka*,
thus creating an fenced off area smaller than Beis Se'asayim, in which it is
permitted to carry.
Rava held that the presence of the Avarneka enclosure makes the *entire
area* that lies before it (i.e. the orchard) considered Hukaf l'Dirah, and
therefore the poles were unnecessary. The Gemara relates that the next day,
Amora'im (including those who had originally been on his side) proved to
Rava that even though the Avarneka was built for residential purposes, it
cannot make the entire orchard Hukaf l'Dirah since it was built *after* the
orchard was completed.
(It is not clear why Rava did not reason like that by himself. Apparently,
he reasoned that since the orchard was subordinate to the Avarneka, it
should be drawn after it. The walls of the *Avarneka* were certainly built
after the *Avarneka* was set aside for Dirah, and thus it itself was Hukaf
l'Dirah. Even though the walls of the orchard itself were there before the
Avarneka, Rava thought the entire orchard was secondary to the Avarneka,
therefore it could not be compared to the usual case of "Hukaf u'l'va'Sof
Yashav.")
The Gemara concludes that the outer Mechitzos themselves must be erected for
the sake of residential purposes in order for the area to be considered
Hukaf l'Dirah.
(c) RABEINU NESANEL (cited by Tosfos, the Ritva and others) also explains
that Rava made a mistake and Rav Huna bar Chinena was correct. Rav Huna bar
Chinena erected a row of poles *along the entire length of one wall of the
orchard*, effectively re-enclosing the orchard *after* the Avarneka was
erected. He thereby made the *entire orchard* into an area that is Hukaf
l'Dirah in which it is permitted to carry.
(Why did Rava think that it was enough to rely on the Avarneka? Tosfos
explains Rava's reasoning as follows. Rava maintained that the only time
that "Hukaf u'l'va'Sof Yashav" does not help to make an area Hukaf l'Dirah
is when a *wall broke down*, allowing passage between the house and the
orchard. Since both the house and the orchard were there before, the
breaching of the wall does not make the orchard Hukaf l'Dirah. However, in
the case of the Avarneka, an entirely *new house* (the Avarneka) was built
after the wall of the orchard was erected. Building a new house, Rava
thought, can make the area considered Hukaf l'Dirah, even if it comes after
the wall.)