To me that seems like odd use of the '{ }' notation.
Ordinarily, = {A | Ex(x in C & A in {Px})} = {A | Ex(x in C & A = Px)}.
1. The problem statement, all variables and given/known data
Prove that
2. Relevant equations
is the powerset of . i.e.
3. The attempt at a solution
Suppose . Then,
Therefore,
Therefore,
But I think there might be something wrong with my proof. Because why can't I start assuming and go to . That means and therefore also. That means
Is there something wrong with this proof?