procedure power_set (s)
  if *s = 0
    then suspend set ()
    else {
      head := set(?s)
      every ps := power_set (s -- head) do {
        suspend ps
        suspend ps ++ head
      }
    }
end

procedure main ()
  every s := power_set (set(1,2,3,4)) do { # power_set's values are generated by 'every'
    writes ("[ ")
    every writes (!s || " ")
    write ("]")
  }
end