code: func (mc *xlsxMergeCells) getExtent(cellRef string) (int, int, error) {
  	if mc == nil {
  		return 0, 0, nil
  	}
  	for _, cell := range mc.Cells {
  		if strings.HasPrefix(cell.Ref, cellRef+cellRangeChar) {
  			parts := strings.Split(cell.Ref, cellRangeChar)
  			startx, starty, err := GetCoordsFromCellIDString(parts[0])
  			if err != nil {
  				return -1, -1, err
  			}
  			endx, endy, err := GetCoordsFromCellIDString(parts[1])
  			if err != nil {
  				return -2, -2, err
  			}
  			return endx - startx, endy - starty, nil
  		}
  	}
  	return 0, 0, nil
  }
docs: // Return the cartesian extent of a merged cell range from its origin
  // cell (the closest merged cell to the to left of the sheet.