More on the contradictory cases where a positive commandment overrides a negative commandment that has a punishment of karet - until that very line of inquiry is upended and rejected as a principle. So the Gemara takes another stab at explaining the rationale behind yibum and the "extra" word of "aleha" in the verse about yibum. Which brings us to hermeneutical derivation of k'lal u-p'rat u-k'lal. Also, how does the logic actually pan out? The Gemara illustrates it with another case - a metzora and the Temple... Which carries some complicated ritual to be able to reenter the camp... But what happens if that same person becomes a baal keri, just in time to not be able to bring the Korban Pesach... Now what?!