A firstborn that protests, his protest is valid. There is a debate among the Rashbam and Rabbeinu Chananel, what he is protesting. Limitations are put on this halacha by Rabba. If a firstborn gives up his rights to a double portion when dividing a particular property, Rav Pappa and Rav Pappi debate (based on a situation where Rava gave a psak about in a different case) whether Rava held that he gave up rights to the double portion of all the properties or only of that particular property? This debate is based on whether one holds that the firstborn gains rights to his double portion immediately upon the death of the father, even before the land is divided or whether that he gains rights to it only once the property is divided. The mishna differentiates between a person who says that he will not bequeath the double portion to his firstborn to a person who says he will equally divide his portion. The first is not allowed as it goes against what it says in the Torah and the second is allowed because it is viewed as a gift. One needs to use the language of a gift in order to bypass Torah laws of inheritance. What type of proof can be used to prove one is the firstborn in order to get the double portion?