 The model of TST that we use contains one type 0 object \(u\). Note that this means that each type is finite. Objects of each type are construed as better and better approximations to the untyped objects of the final set theory. \(u\) approximates any set. The type \(n + 1\) approximant to any set \(A\) is intended to be the set of type \(n\) approximants of the elements of \(A\).