public function add($identifier, $title)
    {
        $identifier = Inflector::slug($identifier);
        $this->_variations[$identifier] = [
            'title' => $title,
            'cfgs' => [],
            'vars' => [],
            'extras' => [],
            'is_default' => false,
        ];
        $this->_tempIdentifier = $identifier;
        return $this;
    }