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A Davar she'Yesh Lo Matirin such as Tevel or Ma'aser Sheni, Hekdesh and Chadash is not Batel. (1)
A Davar she'Ein Lo Matirin such as Terumah, Terumas Ma'aser, Chalah, Orlah and Kil'ei ha'Kerem is Batel.
If produce of Shevi'is gets mixed in with other produce of the same type prior to Bi'ur it is forbidden.
If produce of Shevi'is gets mixed in with other produce after Bi'ur it is Batel.
Onions of the sixth year upon which rain fell on Shevi'is and they sprouted, if the leaves blackened they are forbidden. (2)
Onions of Shevi'is that the rain fell on them in the eighth year and they sprouted if the leaves blackened they are permitted. (3)
Someone who is pruning the garlic of a Kusi may eat the garlic an Achilas Arai without separating Ma'aser. (4)
One must separate definite Ma'aser from produce that is bought from a Kusi.
Produce of Shevi'is that grows in the eighth year, the growth of the eighth year is Mevatel the Isur.
If a person plants a Litra of Ma'aser Tevel and it grows to ten Litrin he must separate Ma'aser on the entire ten Litrin. (6)
A BIT MORE
1. If it is gets mixed in with the same Min it is not Batel, however if it gets mixed in with a different Min it is Batel.
2. Rebbi Chanina Ben Antigonus says that if it could be pulled out of the ground by its leaves it is forbidden.
3. The growth of the eighth year is Mevatel the Isur of Shevi'is.
4. A person may eat an Achilas Arai without separating Ma'aser if the Gmar Melachah on the produce has yet to be completed.
6. He must separate Terumas Ma'aser on the original Litra from another place.
K'HETERO KACH ISURO
The Beraisa says that Tevel is a Davar she'Yesh Lo Matirin and therefore it is not Batel The Ran says that the Gemara in Avodah Zarah gives a different reason why Tevel is not Batel, because k'Hetero Kach Isuro – just as the Heter of Tevel is with one kernel of wheat, because Min ha'Torah a kernel of wheat is enough for Terumah, so too one kernel of wheat of Isur is Chashuv enough to prohibit the entire mixture. The Netziv explains that the reason of k'Hetero Kach Isuro only applies to Tevel that the Terumah was taken from it, but not to Tevel that the Ma'aser or Terumas Ma'aser was not separated from it. This is because only with regards to Terumah is one kernel enough but Ma'aser and Terumas Ma'aser require 10%, therefore we also need the reason of Davar She'Yesh Lo Matirin.
AN EGG ON YOM TOV
If an egg is laid on Yom Tov it is forbidden to touch it and certainly it is forbidden to eat it. If it gets mixed up [with other eggs] even in a thousand they are all forbidden. If it is a Safek whether it was laid on Yom it is forbidden. (Shulchan Aruch OC 513:1, 2)
The egg is not Batel even in a thousand because it will be permitted after Yom Tov and everything that will be permitted is not Batel even in a thousand. For that reason also it is forbidden when it is a Safek whether it was laid on Yom Tov even though normally a Safek mid'Rabanan is permitted. (Mishnah Berurah)
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