function start_output() {
        $this->started_output = true;
        if ($this->exportclass!==null) {
            $this->exportclass->start_table($this->sheettitle);
            $this->exportclass->output_headers($this->headers);
        } else {
            $this->start_html();
            $this->print_headers();
            echo html_writer::start_tag('tbody');
        }
    }