code: private static String key2Str(Key k, byte type) {
      String s = key2Str_impl(k, type);
      Key x;
      assert (x = str2Key_impl(s)).equals(k) : "bijection fail " + k + "." + (char) type + " <-> " + s + " <-> " + x;
      return s;
    }
docs: Verify bijection of key/file-name mappings.