@Override
    public boolean add(E e) {
        return map.putIfAbsent(e, e) == null;
    }
@Test
    public void testAddFailure() {
        ExtendedSet<TestValue> set = new ExtendedSet<>(Maps.newConcurrentMap());
        TestValue val = new TestValue("foo", 1);
        assertTrue(set.add(val));
        assertFalse(set.add(val));
    }