Ruzsa (“ROO-zhah”) names a distance function between these objects that can be used to prove a bound on sums of these objects called Plünnecke’s inequality. Functors from a locally small category into the category of these objects are the subject of the Yoneda lemma. (10[1])A class is called “proper” if it is “too big” to be expressed as one of these objects. (10[1])If X is one of these objects, then X has size strictly smaller than that of “two to the X” by Cantor’s theorem. (10[2])Problems with constructing these objects via (10[1])unrestricted comprehension (10[1])were resolved with a theory of these objects named for (10[1])Zermelo (10[1])and Fraenkel (10[1])that was later (10[1])extended to include the axiom of choice. Countably infinite instances of these objects have cardinality equal to that of the natural numbers. For 10 points, name these unordered collections of elements. ■END■