public function addColumn($fieldsets, $class = null)
    {
        if (!is_array($fieldsets)) {
            $fieldsets = [$fieldsets];
        }
        foreach ($fieldsets as $i => $num) {
            $fieldsets[$i] = (int)$num - 1;
        }
        if (null === $class) {
            $class = 'pop-form-column-' . (count($this->columns) + 1);
        }
        $this->columns[$class] = $fieldsets;
        return $this;
    }