@Override
    public ImmutableSet<E> added(E e) {
        return new PCollectionsImmutableSet<>(underlying().plus(e));
    }
@Test
    public void testDelegationOfAdded() {
        new PCollectionsHashSetWrapperDelegationChecker<>()
            .defineMockConfigurationForFunctionInvocation(mock -> mock.plus(eq(this)), SINGLETON_SET)
            .defineWrapperFunctionInvocationAndMockReturnValueTransformation(wrapper -> wrapper.added(this), identity())
            .expectWrapperToWrapMockFunctionReturnValue()
            .doFunctionDelegationCheck();
    }