@Test
 public void powerSet() {
   MutableSet<String> set =
       MultiReaderUnifiedSet.newSetWith("1", "2", "3", "4").asReadUntouchable();
   MutableSet<UnsortedSetIterable<String>> powerSet = set.powerSet();
   Verify.assertSize((int) StrictMath.pow(2, set.size()), powerSet);
   Verify.assertContains(UnifiedSet.<String>newSet(), powerSet);
   Verify.assertContains(set, powerSet);
 }