sub add_html {
        my $self = shift;
        my $ref = shift;

        # step over elements in fixed order
        my @elements;
        foreach my $key (qw(loc changefreq lastmod priority)){
                if($key eq 'loc'){
                        if($ref->{loc}){
                                push(@elements, XML::Twig::Elt->new('loc', {}, $ref->{loc}) );
                        }else{
                                print "Error: 'loc' is a mandatory value and missing!\n"
                        }
                }elsif($key eq 'priority'){
                        if($ref->{priority}){
                                push(@elements, XML::Twig::Elt->new('priority', {}, $ref->{priority}) );
                        }else{
                                print "Error: 'priority' is a mandatory value and missing!\n"
                        }
                }
        }

        # wrap these sub-elements into an "url"-level
        my $elt = XML::Twig::Elt->new('url', {}, @elements);
        undef(@elements);

        # and add this url-sub-element nest to the xml-document
        $elt->paste(last_child => $self->{xml});

        $self->{cnt}->{html}++;
}