@Test
 public void isProperSubsetOf() {
   MutableSet<String> set =
       MultiReaderUnifiedSet.newSetWith("1", "2", "3", "4").asReadUntouchable();
   Assert.assertTrue(set.isProperSubsetOf(UnifiedSet.newSetWith("1", "2", "3", "4", "5")));
   Assert.assertFalse(set.isProperSubsetOf(set));
 }