public Collection<ConfigEntry> entries() {
        return Collections.unmodifiableCollection(entries.values());
    }
@Test
    public void shouldImplementEqualsProperly() {
        assertEquals(config, config);
        assertEquals(config, new Config(config.entries()));
        assertNotEquals(new Config(Collections.singletonList(E1)), config);
        assertNotEquals(config, "this");
    }