A SetOrClass that satisfies extensionality as well as other constraints specified by some choice of set theory. Sets differ from Classes in two important respects. First, Sets are extensional - two Sets with the same elements are identical. Second, a Set can be an arbitrary stock of objects. That is, there is no requirement that Sets have an associated condition that determines their membership. Note that Sets are not assumed to be unique sets, i.e. elements of a Set may occur more than once in the Set.