@Override
    public int hashCode() {
        return underlying().hashCode();
    }
@Test
    public void testHashCode() {
        final MapPSet<Object> mock = mock(MapPSet.class);
        assertEquals(mock.hashCode(), new PCollectionsImmutableSet<>(mock).hashCode());
        final MapPSet<Object> someOtherMock = mock(MapPSet.class);
        assertNotEquals(mock.hashCode(), new PCollectionsImmutableSet<>(someOtherMock).hashCode());
    }