new IntegerDiv10(12), "twelve");

    assertThrows(IllegalArgumentException.class, () -> ImmutableSortedMap.copyOf(original));
  }

  public void testImmutableMapCopyOfImmutableSortedMap() {
    IntegerDiv10 three = new IntegerDiv10(3);
    IntegerDiv10 eleven = new IntegerDiv10(11);
    IntegerDiv10 twelve = new IntegerDiv10(12);
    IntegerDiv10 twenty = new IntegerDiv10(20);