Another Requirement There is also an additional requirement on sets:A non-empty set must contain an element disjoint from itself. ·Formally,V set A卡p,3x∈Ast.xnA=p. "This helps us to rule out many strange "sets"-they are not sets,but proper classes. ■Examples: ■{{…}}is not a set. There are no sets A and B such that A∈B and B∈A. (Why?)Another Requirement  There is also an additional requirement on sets: A non-empty set must contain an element disjoint from itself.  Formally,  This helps us to rule out many strange “sets”- they are not sets, but proper classes.  Examples:  {{{…}}} is not a set.  There are no sets A and B such that (Why?)