public static function getInstance()
 {
     if (!self::$instance instanceof self) {
         self::$instance = new self();
     }
     return self::$instance;
 }
 /**
  * @dataProvider provider
  */
 public function testFlattening($data, $result)
 {
     $this->assertEquals($result, $this->flattenValuesCommand->convert($data));
 }