@Override
    public String toString() {
        return (value == null) ? "(null)" : '"' + getValue() + '"';
    }
@Test
    void testUnresolvedReference() {
        var reference = ModelReference.unresolved(Optional.of("myModelId"),
                                                  Optional.of(new UrlReference("https://host:my/path")),
                                                  Optional.of(new FileReference("foo.txt")));
        assertEquals("myModelId https://host:my/path foo.txt", reference.toString());
        assertEquals(reference, ModelReference.valueOf(reference.toString()));
    }