code: public function generateNavigation()
      {
          // Spreadsheet object known?
          if ($this->spreadsheet === null) {
              throw new WriterException('Internal Spreadsheet object not set to an instance of an object.');
          }
  
          // Fetch sheets
          $sheets = [];
          if ($this->sheetIndex === null) {
              $sheets = $this->spreadsheet->getAllSheets();
          } else {
              $sheets[] = $this->spreadsheet->getSheet($this->sheetIndex);
          }
  
          // Construct HTML
          $html = '';
  
          // Only if there are more than 1 sheets
          if (count($sheets) > 1) {
              // Loop all sheets
              $sheetId = 0;
  
              $html .= '<ul class="navigation">' . PHP_EOL;
  
              foreach ($sheets as $sheet) {
                  $html .= '  <li class="sheet' . $sheetId . '"><a href="#sheet' . $sheetId . '">' . $sheet->getTitle() . '</a></li>' . PHP_EOL;
                  ++$sheetId;
              }
  
              $html .= '</ul>' . PHP_EOL;
          }
  
          return $html;
      }
docs: Generate sheet tabs.
  
  @throws WriterException
  
  @return string