public function testCanCompareDifferentIdentifier()
 {
     $identifier1 = new ArticleIdentifier('test1');
     $identifier2 = new ArticleIdentifier('test2');
     $this->assertFalse($identifier1->equals($identifier2));
 }
 /**
  * @return bool
  */
 public function equals(ArticleIdentifier $articleIdentifier) : bool
 {
     return $this->identifier() === $articleIdentifier->identifier();
 }