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 (10[1])inequality. (10[1])Functors from a locally small category into the category of these objects (10[1])are (10[1])the subject of the Yoneda lemma. (10[5])A class (10[1])is called “proper” (10[1])if it is “too big” (10[2])to (10[1])be expressed (10[2])as one of (10[1])these objects. (10[6])If X (10[1])is one of these objects, (10[1])then X has size strictly smaller than that of “two to the X” (10[1])by Cantor’s (10[2])theorem. (10[9]-5[1])Problems (10[1])with (10[1])constructing (10[3])these objects (10[2])via (10[3])unrestricted (10[1])comprehension (10[2]-5[1])were resolved (10[2])with a theory of these objects (10[2])named for (10[1])Zermelo (10[4])and (10[2])Fraenkel (10[14])that (10[2])was later (10[2]-5[2])extended (10[4])to (10[1])include the axiom of choice. (10[4])Countably (-5[1])infinite instances (10[1])of these objects (10[2])have (10[1])cardinality (10[1])equal to that (10[1])of the natural numbers. (10[1]-5[1])For 10 points, name these unordered collections (10[1])of elements. (10[4])■END■ (10[8]0[4])