A SeT is a collection of objects. For example, one can define the set **S** = {Sn: Sn is a sibling of the Larry M. Sanger, who is the Editor-in-Chief of Nupedia}.

We require that sets be well-defined. Given an object Sn, we must be able to determine if
Sn belongs to **S**.

**What, there are no recursively enumerable sets?**