For questions about the surreal numbers, an inductively constructed ordered field that naturally contains all ordinal numbers.

The surreal numbers are an inductively constructed proper class which has the structure of an ordered field. Surreal numbers were originally discovered in the context of combinatorial game theory, as they form a very special class of "games" in the sense of combinatorial game theory. For more information, see https://en.wikipedia.org/wiki/Surreal_number.