has 'next_document',     is => 'rw';
has 'prev_document',     is => 'rw';

my $prev;

        # link documents in traversal order
        my $doc = $parser->get_document;
        if ($prev)
        {
            $prev->next_document( $doc );
            $doc->prev_document( $prev );
        }

        $prev = $doc;
        push @docs, $doc;
