code: public function resetStart($startColumn = 'A')
      {
          $startColumnIndex = Coordinate::columnIndexFromString($startColumn);
          if ($startColumnIndex > Coordinate::columnIndexFromString($this->worksheet->getHighestColumn())) {
              throw new Exception("Start column ({$startColumn}) is beyond highest column ({$this->worksheet->getHighestColumn()})");
          }
  
          $this->startColumnIndex = $startColumnIndex;
          if ($this->endColumnIndex < $this->startColumnIndex) {
              $this->endColumnIndex = $this->startColumnIndex;
          }
          $this->seek($startColumn);
  
          return $this;
      }
docs: (Re)Set the start column and the current column pointer.
  
  @param string $startColumn The column address at which to start iterating
  
  @throws Exception
  
  @return ColumnIterator