public function testIdentifierCanBeRetrieved()
 {
     $identifierString = "test";
     $articleIdentifier = new ArticleIdentifier($identifierString);
     $this->assertEquals($identifierString, $articleIdentifier->identifier());
 }
 /**
  * @return bool
  */
 public function equals(ArticleIdentifier $articleIdentifier) : bool
 {
     return $this->identifier() === $articleIdentifier->identifier();
 }