protected function tableHeaders() : string
    {
        $arrCells = [];
        foreach ($this->getHeaderColumns() as $key => $val) {
            $arrCells[] = $this->renderHeaderColumn($key, $val);
        }        
        $arrCells[] = "";
        
        $content = H::renderTableHeaderRow($arrCells);
        $full = H::thead($content)
                . H::tbody('');
        return $full;
    }