code: public static function getInstance(Spreadsheet $spreadsheet = null)
      {
          if ($spreadsheet !== null) {
              $instance = $spreadsheet->getCalculationEngine();
              if (isset($instance)) {
                  return $instance;
              }
          }
  
          if (!isset(self::$instance) || (self::$instance === null)) {
              self::$instance = new self();
          }
  
          return self::$instance;
      }
docs: Get an instance of this class.
  
  @param Spreadsheet $spreadsheet Injected spreadsheet for working with a PhpSpreadsheet Spreadsheet object,
  or NULL to create a standalone claculation engine
  
  @return Calculation