@Override
    public boolean isSet(int index) {
        long bit = 1L << index;
        return (mask & bit) == bit;
    }
@Test
  public void testIsSet() {
      assertThat(new LongBitMask().set(1).isSet(0)).isFalse();
      assertThat(new LongBitMask().set(1).isSet(1)).isTrue();
  }