public static function registerLoader($loader, string ...$contentTypes): FluentDOM\Loaders {
    $loaders = self::getDefaultLoaders();
    if (count($contentTypes) > 0) {
      $lazyLoader = new \FluentDOM\Loader\Lazy();
      foreach ($contentTypes as $contentType) {
        $lazyLoader->add($contentType, $loader);
      }
      $loaders->add($lazyLoader);
    } elseif (is_callable($loader)) {
      self::registerLoader($loader());
    } else {
      $loaders->add($loader);
    }
    self::$_loader = NULL;
    return $loaders;
  }