protected function getUsedColumns()
    {
        if (strlen($this->maxColumn) == 2) {
            $usedColumns = range('A', 'Z');
            $max = $this->maxColumn[0];
            $upperRange = range('A', $max);
            foreach ($upperRange as $x) {
                $end = $max == $x ? $this->maxColumn[1] : 'Z';

                foreach (range('A', $end) as $y) {
                    $usedColumns[] = sprintf('%s%s', $x, $y);
                };
            }
        } else {
            $usedColumns = range('A', $this->maxColumn);
        }

        return $usedColumns;
    }