public function __construct(Identifier $id)
 {
     parent::__construct($id);
 }
 public function __construct(Identifier $id, Identifier $item_id)
 {
     $this->item_id = $item_id;
     parent::__construct($id);
 }
 public function __construct(Identifier $id, Identifier $item_id, Quanitity $quantity)
 {
     $this->item_id = $item_id;
     $this->quantity = $quantity;
     parent::__construct($id);
 }
 public function __construct(Identifier $id, Identifier $customer_id)
 {
     $this->customer_id = $customer_id;
     parent::__construct($id);
 }