code: protected function fileIterator($directoryPath)
      {
          $this->typeCheck->fileIterator(func_get_args());
  
          return new RecursiveIteratorIterator(
              new RecursiveDirectoryIterator(
                  $directoryPath,
                  FilesystemIterator::FOLLOW_SYMLINKS |
                  FilesystemIterator::SKIP_DOTS
              )
          );
      }
docs: @param string $directoryPath
  
  @return RecursiveIteratorIterator