return value / 10 - o.value / 10;
    }

    @Override
    public String toString() {
      return Integer.toString(value);
    }
  }

  public void testCopyOfDuplicateKey() {
    Map<IntegerDiv10, String> original =
        ImmutableMap.of(
            new IntegerDiv10(3), "three",
            new IntegerDiv10(20), "twenty",
            new IntegerDiv10(11), "eleven",