The gemara says on 17b that the reason why rava said that whoever gives birth should daven that their child should be like reb shimon is because given his own line of reasoning that each one is more chamur than the other he changed the whole order of the passuk, because had reb shimon had gone in order of the passuk and we would come to bechor we wouldn't be able to say that it's coming for after zerika because we don't even know the stage of Lifnei Zerikah, so we would say that the passuk of bechor is coming to tell us that there is a lav before zerika and we wouldn't know that there is a lav l'Achar Zerika by bechor.(and what we are trying to do is to look for a lav of l'Achar Zerika by bechor since we already have an Aseh of u'Vesaram Yihyeh Lecha k'Chazeh ha'Tenufah u'k'Shok ha'Terumah as Rashi says on 17a in DH Afilu l'Achar Zerikah because only once we have an aseh can we go ahead and make it into a lav because we can't just go ahead and make up a lav by bechor, only once we have an aseh by bechor then can we make it into a lav)
So now my question is that even if we would go in order how would we even be able to say that bechor is teaching us a lav before zerika, but we first need to have an aseh which tells us that you can only eat it after zerika which is the passuk of v'Dam Zevachecha Yishafech ... as Rashi says on 17a in DH Lifnei Zerikah. However this passuk is not by a bechor. So it comes out that we don't even have an aseh which we would be able to make it into a lav so how could we even be able to establish a lav Lifnei Zerikah by a bechor if we were to go In order of the passuk?
Benzi, London
We do not need an aseh which we go ahead to make into a lav. It is possible for there to ba a lav even if there is not an aseh to build on. Rashi writes a few times on 17a that up to now we only had an aseh, but this does not mean that you cannot make a lav if you do not have an aseh. What Rashi was worried about was why do we need another posuk for a Mitzvah that we already know about and he answered that up to now we only had an aseh but now we have a lava as well.
A Gezunter Zummer
Dovid Bloom
But why does Rashi get so busy with trying to find an aseh aswell but the main thing of our gemara is to find a lav not an aseh?
Possibly Rashi wants to tell us that there are a lot of prohbitions involved, in a similar way that the Mishnah below top 21b says that a person can transgess 8 lavin just by ploughing one furrow.
Chodesh TOV
Dovid Bloom