public IEnumerable<IEnumerable<T>> GetPowerSet<T>(IEnumerable<T> input) {
    var seed = new List<IEnumerable<T>>() { Enumerable.Empty<T>() }
      as IEnumerable<IEnumerable<T>>;

    return input.Aggregate(seed, (a, b) =>
      a.Concat(a.Select(x => x.Concat(new List<T>() { b }))));
  }