sub spsqs # single pass stable quicksort
  {
  @_ < 2 and return @_;
  my ($pivot, @items) = ($_[@_ / 2], [], [], []);
  push $items[$_ <=> $pivot]->@*, $_ for @_;
  spsqs($items[-1]->@*), $items[0]->@*, spsqs($items[1]->@*)
  }