An F# list can be thought of as representing a set, where the order of the elements in the list is irrelevant.

Write an F# function powerset such that powerset of set S returns the set of all subsets of S. For example,

> powerset [1;2;3];;

val it : int list list

= [[]; [3]; [2]; [2; 3]; [1]; [1; 3]; [1; 2]; [1; 2; 3]]

Note that you can order the elements of the powerset however you wish.