public function testDefaultPersistentCollectionGenerator()
 {
     $c = new Configuration();
     $c->setPersistentCollectionDir(__DIR__ . '/../../../../PersistentCollections');
     $c->setPersistentCollectionNamespace('PersistentCollections');
     $generator = $c->getPersistentCollectionGenerator();
     $this->assertInstanceOf(PersistentCollectionGenerator::class, $generator);
     $this->assertSame($generator, $c->getPersistentCollectionGenerator());
 }