Description: Define power class. Definition 5.10 of TakeutiZaring p. 17, but we
also let it apply to proper classes, i.e. those that are not members of
_V . When applied to a set, this produces its power set. A power
set of S is the set of all subsets of S, including the empty set and S
itself. For example, if A = { 3 , 5 , 7 } , then
~P A = { (/) , { 3 } , { 5 } , { 7 } , { 3 , 5 } ,{ 3 , 7 } , { 5 , 7 } , { 3 , 5 , 7 } } ( ex-pw ). We will later
introduce the Axiom of Power Sets ax-pow , which can be expressed in
class notation per pwexg . Still later we will prove, in hashpw ,
that the size of the power set of a finite set is 2 raised to the power
of the size of the set. (Contributed by NM, 24-Jun-1993)