new EqualsTester()
          .addEqualityGroup(powerSet1, powerSet2, toHashSets(powerSet1))
          .addEqualityGroup(ImmutableSet.of())
          .addEqualityGroup(ImmutableSet.of(9999999))
          .addEqualityGroup("notASet")
          .testEquals();
      assertEquals(toHashSets(powerSet1).hashCode(), powerSet1.hashCode());
    }
  }