sub derangement {
  my $list = shift;
  my $i = @$list;
  my @swapped;

  while (--$i) {
    my $j = int rand ($i+1);
    redo if $i == $j and !$swapped[$i];
    @$list[$i,$j] = @$list[$j,$i];
    @swapped[$i,$j] = (1,1);
  }
}