private function expandShorthandDefinitions()
    {
        foreach ($this->mapping as $key => $settings) {
            if (is_string($key) && is_string($settings)) {
                $this->mapping[$key] = [
                    'type' => 'column',
                    'mapping' => [
                        'destination' => $settings
                    ]
                ];
            }
        }
    }