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1. The Gemara explains how we understand that the words, "Pri Etz Hadar" -- "the fruit of a Hadar tree," refer to an Esrog.
2. The Gemara quotes other opinions for how we know that the verse (#1) refers to an Esrog.
3. The Gemara explains why one may not use an Esrog of idolatry.
4. There is a dispute about whether one may fulfill the Mitzvah with an Esrog of Ma'aser Sheni.
5. There is a dispute about whether one may fulfill the Mitzvah with an Esrog of Demai.
A BIT MORE
1. This verse refers to a fruit of a tree the wood of which tastes similar to the fruit, which is an Esrog. The Gemara proceeds to ask why it does not refer to other fruit that have this characteristic.
2. For example, Rebbi Avahu says that "Hadar" alludes to a fruit that lives ("ha'Dar") on its tree from year to year, which describes an Esrog tree.
3. Since it is supposed to be burned, it is as if it already lacks the proper size needed to be a valid Esrog (it is like a small pile of ashes).
4. Rebbi Meir: One cannot fulfill the Mitzvah in this manner. Chachamim: One may fulfill the Mitzvah with such an Esrog.
5. Beis Hillel: He may fulfill the Mitzvah with an Esrog of Demai, since, if he wanted, he could declare his property ownerless and then be eligible to eat it, as poor people may eat Demai. Beis Shamai: He may not fulfill the Mitzvah with it, since even poor people are not allowed to eat Demai.
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