sub next_multiset {
  my $self = shift;

  if ( ! $self->{frequency} ) {
    print STDERR "must use next_combination() if 'frequency' argument not passed to constructor\n";
    return ();
  }

  my $data = $self->data();
  my $compare = $self->compare();

  while ( my @combo = $self->_next_combination ) {
    my $x = join '', map {scalar($$_)} sort @$data;
    my $y = join '', map {scalar($_) } sort @combo;

    next if $self->{'cache_multiset'}{$y}++;
    return @combo;
  }
  $self->{'cache_multiset'} = undef;
  return ();
}