code: public ISO9660Directory addDirectory(ISO9660Directory dir) {
          dir.setLevel(level + 1);
          dir.setParentDirectory(this);
          dir.setRoot(root);
          updateSubdirectories(dir);
          directories.add(dir);
          sorted = false;
          return dir;
      }
docs: Add directory
  
  @param dir Directory to be added
  
  @return Added directory