protected function createNamedSubDirectory(core_kernel_file_File $rootDirectory, $activity)
 {
     $name = md5($activity->getUri());
     $relPath = $rootDirectory->getRelativePath() . DIRECTORY_SEPARATOR . $name;
     $absPath = $rootDirectory->getAbsolutePath() . DIRECTORY_SEPARATOR . $name;
     if (!is_dir($absPath) && !mkdir($absPath)) {
         throw new taoItems_models_classes_CompilationFailedException("Could not create sub-directory '{$absPath}'.");
     }
     return $rootDirectory->getFileSystem()->createFile('', $relPath);
 }