code: public function prev()
      {
          if ($this->currentColumnIndex <= $this->startColumnIndex) {
              throw new PhpSpreadsheetException('Column is already at the beginning of range (' . Coordinate::stringFromColumnIndex($this->endColumnIndex) . ' - ' . Coordinate::stringFromColumnIndex($this->endColumnIndex) . ')');
          }
          do {
              --$this->currentColumnIndex;
          } while (($this->onlyExistingCells) && (!$this->worksheet->cellExistsByColumnAndRow($this->currentColumnIndex, $this->rowIndex)) && ($this->currentColumnIndex >= $this->startColumnIndex));
      }
docs: Set the iterator to its previous value.
  
  @throws PhpSpreadsheetException