public function group($fields, $overwrite = false)
    {
        if ($overwrite) {
            $this->_parts['group'] = [];
        }

        if (!is_array($fields)) {
            $fields = [$fields];
        }

        $this->_parts['group'] = array_merge($this->_parts['group'], array_values($fields));
        $this->_dirty();

        return $this;
    }