assertEquals(i, powerSet(newHashSet(elements)).size());
  }

  private static void checkHashCode(Set<?> set) {
    assertEquals(new HashSet<>(set).hashCode(), set.hashCode());
  }

  public void testCombinations() {
    ImmutableList<Set<Integer>> sampleSets =
        ImmutableList.<Set<Integer>>of(
            ImmutableSet.<Integer>of(),
            ImmutableSet.of(1, 2),