@Nullable
    public String execute() {
      while (true) {
        String commonHead = getCommonFragment(myIndex);
        if (commonHead == null) break;
        append(commonHead, myBuffer);
        mySeparator = File.separator;
        myIndex++;
      }
      return myIndex > 0 ? myBuffer.toString() : null;
    }
@Nullable
    public String execute()