void main() {
  List<int> list = [1, 2, 3];
  print(subs(list));
}

List<List<A>> subs<A>(List<A> list) => /*
         checkingOrder={List<A>,<A>[],<A>[(), ...]},
         subtypes={<A>[],<A>[(), ...]},
         type=List<A>
        */ switch (list) {
  [] /*space=<[]>*/ => [],
  [var x, ...var xs] /*space=<[Object?, ...List<A>]>*/ => [
    for (var ys in subs(xs)) ...[
      [x] + ys,
      ys,
    ],
    [x],
  ],
};
