code: public function getConfigTreeBuilder()
      {
          $treeBuilder = new TreeBuilder();
          $rootNode = $treeBuilder->root($this->root);
  
          $classDefinition = $rootNode
              ->children()
                  ->arrayNode('classes')
                      ->prototype('array')
                          ->children();
  
          $this->appendClassDefinition($classDefinition);
  
          $classDefinition->end()
                      ->end()
                  ->end()
              ->end();
  
          return $treeBuilder;
      }
docs: {@inheritdoc}
  @throws \RuntimeException
  @throws \InvalidArgumentException