code: public function freezePane($cell, $topLeftCell = null)
      {
          if (is_string($cell) && Coordinate::coordinateIsRange($cell)) {
              throw new Exception('Freeze pane can not be set on a range of cells.');
          }
  
          if ($cell !== null && $topLeftCell === null) {
              $coordinate = Coordinate::coordinateFromString($cell);
              $topLeftCell = $coordinate[0] . $coordinate[1];
          }
  
          $this->freezePane = $cell;
          $this->topLeftCell = $topLeftCell;
  
          return $this;
      }
docs: Freeze Pane.
  
  Examples:
  
  - A2 will freeze the rows above cell A2 (i.e row 1)
  - B1 will freeze the columns to the left of cell B1 (i.e column A)
  - B2 will freeze the rows above and to the left of cell B2 (i.e row 1 and column A)
  
  @param null|string $cell Position of the split
  @param null|string $topLeftCell default position of the right bottom pane
  
  @throws Exception
  
  @return Worksheet