sub intersection_1
{
  my $count = shift;
  $count--;
  my %saw;

  grep($count == $saw{$_}++, @_);
}

# This code is based on the SortedMultiList code but works
# for an intersection of more than two sets.
sub intersection_2
{
  my $self = [@_];
  my ($lowest_arr, $lowest, $next);
  my @ret = ();
  my $elements = scalar @_;

  while (1) {
    my $count = 0;
    $lowest = $next;
    if (!defined($lowest)) {
      # find min element in queue
      foreach my $a (@$self)
      {
        return @ret unless (@$a);
        if (!defined($lowest) or ($a->[0] lt $lowest))
        {
          $lowest = $a->[0];
        }
      }
    }
    foreach my $a (@$self)
    {
      return @ret unless (@$a);
      if ($a->[0] eq $lowest)
      {
         shift @$a;
         $count++;
      } else {
        if (!defined($next) or ($a->[0] lt $next))
        {
          $next = $a->[0];
        }
      }
    }
    if ($count == $elements) {
        push(@ret, $lowest);
    }
    $lowest = $next;
    undef $next;
  }
}