@Override
    public Object getSingleValue()
    {
        if (!isSingleValue()) {
            throw new IllegalStateException("EquatableValueSet does not have just a single value");
        }
        return entries.iterator().next().getValue();
    }
@Test
    public void testGetSingleValue()
    {
        assertEquals(EquatableValueSet.of(TestingIdType.ID, 0L).getSingleValue(), 0L);
        assertThrows(IllegalStateException.class, () -> EquatableValueSet.all(TestingIdType.ID).getSingleValue());
    }