jewish books - Why is the Rif's digest of tractate Niddah hidden in tractate Shevuos?

Rabbi Isaac Alfasi's summary of tractate Niddah is hidden in masechet Shevuot. Why is it buried in maseches Shevuos?

Answer

It is not all in Shevuot, there is some in Yevamot. I think the reason is that there is only about one or two perakim that are הלכה למעשה (like הרואה כתם) in Niddah, so the Rif wrote down the halachos thereof on masechtos that he was writing on anyway that have sugyas related to Niddah, like the end of the second perek of Shevuot.