private function createExtensionNode(ArrayNodeDefinition $rootNode, $name, $validator)
  {
    $rootNode
      ->children()
      /**/->arrayNode($name)
      /*  */->addDefaultChildrenIfNoneSet(array())
      /*  */->requiresAtLeastOneElement()
      /*  */->useAttributeAsKey('name')
      /*  */->prototype('variable')
      /*    */->validate()
      /*      */->ifTrue($validator)
      /*      */->thenInvalid('Configuration value must be callable')
      /*    */->end()
      /*  */->end()
      /**/->end()
      ->end();
  }