sub mergesort
  {
  @_ < 2 and return @_;
  my @low = mergesort( @_[0 .. $#_ / 2] );
  my @high = mergesort( @_[$#_ / 2 + 1 .. $#_] );
  map !@high || @low && $low[0] <= $high[0] ? shift @low : shift @high, 1..@_;
  }

##</code><code>##

sub sqs # stable quicksort
  {
  @_ < 2 and return @_;
  my $p = $_[@_ / 2];     # pivot
  sqs( grep $_ < $p, @_ ), grep( $_ == $p, @_ ), sqs( grep $_ > $p, @_ );
  }

##</code><code>##

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]->@*)
  }