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