Explanation #1 (Rashi): Three sides enter the corner [of where two walls of the Chatzer meet], and only the fourth side is seen from outside.

Both of them needed to be taught. A corner teaches the extremity of Rav Sheshes, and an irrigation channel, which is [seen] from two sides, teaches the extremity of Rabah, who permits.

However, Rabah agrees when it is near a wall, that it is even with the ground only from one side, that it is forbidden.

Implied question: The Beraisa below says that one side is even with the ground. This is difficult for Rav Sheshes [who forbids even when two sides are even with the ground]!

Answer: He holds that "one" is not precise. The same applies when two sides are even.

Example: [Blood of] an Olah is put twice [on opposite corners of the Mizbe'ach,] which is four (the blood goes on all four sides. It was mixed with blood of) Bechor or Pesach (which is put only once).

Suggestion: "[A Korban whose blood] is put four times" refers to Chatas (its blood is truly put four times, once on each corner)!

Rejection: If [blood of] Chatas became mixed with [blood of] Pesach or Bechor, there is no solution, for [Chatas] must be put above Chut ha'Sikra (a divider at half the height of the Mizbe'ach), and the other must be put below.

Question: The Aseh (to put the blood in the proper place) should override the Lav of Bal Tosif!

Answer: This does not resemble at all Sha'atnez in Tzitzis (from which we learn that Aseh Docheh Lo Sa'aseh). Here (the need to transgress in order to do the Aseh) came through negligence. [Had they not become mixed, the Aseh] could have been fulfilled without overriding the Lav!

Explanation: We hold that any [blood] that must be put [on the outer Mizbe'ach], if it was put once, (he fulfilled his obligation and the Korban) atoned.

Question: Above we forbid dry, for it asks "twigs will fall off...!"

Answer: In a tree, which is tall, it is more appropriate to decree, for it is evident in it [when] twigs fall off.

Explanation (R. Chananel): This Heter is only for grass. It is permitted in every case. However, one cannot prove from here any Heter for a tree, even if it is dry, because there is more reason to decree, because it is evident.