Yevamot 50: How Final Are the Yibum Options?

The fifth chapter! Plus, the options of ma'amar, get, chalitzah... and, of course, yibum. And how the may follow one from the other, as it happens. Note - a long mishnah here establishes the basic cases and basic principles - for example, once ma'amar has been done, it can't be done again in that same context. The Gemara basically establishes where Rabban Gamliel and the sages agree and disagree.