Aaron is correct. The power set of a empty set is not an empty set. You can see the wikipedia https://en.wikipedia.org/wiki/Power_set#Algorithms.

btw, @asmeurer we give a wrong result here
```
In [6]: FiniteSet(EmptySet())
Out[6]: ∅

In [7]: FiniteSet(FiniteSet(EmptySet()))
Out[7]: ∅
```