Computing a set of all subsets (power set)

I am trying to get a function (given as a parameter a set) to return a set whose elements are all the subset formed from the main set.Ex: {1;2;3} -> { {1}, {2}, {3}, {1,2}, {1,3}, {2,3}, {1,2,3} }

But I don't exactly know how to make a module that lets me work with a set of sets. What type should I define it as?