public function header()
    {
        // If we have tab content
        if (isset($this->content_html{0})) {
            // Tab sub-headers html
            $sub_headers = '';

            // Iterate tab group tabs only if there is atleast two sub headers
            if (sizeof($this->tabs) > 2) {
                foreach ($this->tabs as $tab) {
                    // Don't render this tab as sub header
                    if ($tab == $this) continue;

                    // Render tab header as sub-header
                    $sub_headers .= m()->view($this->header_view)->tab($tab)->output();
                }
            }

            // Render just last tab
            $this->header_html .= m()
                ->view($this->header_view)
                ->sub_headers($sub_headers)
                ->class(isset($sub_headers{0}) ? 'sub-tabs-list' : '')
                ->tab($this)// Pass this tab object as main
                ->tab_id(end($this->tabs)->id)// Pass last tabs object id as identifier
                ->output();
        }

        return $this->header_html;
    }