$ENTRY Go {
    = <Prout '[A B C D]:' <Powerset A B C D>>
      <Prout '[]:' <Powerset>>
      <Prout '[()]:' <Powerset ()>>;
};

Powerset {
    = ();
    e.X t.I, <Powerset e.X>: e.F = e.F <Append t.I e.F>;
};

Append {
    t.I = ;
    t.I (e.X) e.Y = (e.X t.I) <Append t.I e.Y>;
};