Let \(A {:} \U\) be a small type, and \(x {:} A\vdash M\) be an indexed family of iterative sets. Then \(\sup(A,x.M)\), or with a more suggestive notation \(\{ M\mid x {:} A\}\), is an iterative set. To paraphrase: an iterative set is a family of iterative sets indexed by a small type.